Possibly this is greatest book defining what is Bhakthi written by Narada who was the greatest among Bhakthas of Lord Narayana.
1.Atha tho Bakthim vyakyasyama
Now I will explain in detail what is meant by devotion to God(Bhakthi).
2.Saa tasmin Parama prema roopa
Devotion to God is personification of intense(highest) love towards Him.
3.Amrutha swaroopa cha
It also is deathless or as sweet as nectar (or it also confers deathlessness to one)
4.Yallabhathva puman siddho bhavathi, amrutho bhavathi, truptho bhavathi
One who gets it becomes perfect, immortal and contended.
5.Yath prapya kinchid vanchadhi , na sochathi, na dweshati, na ramathe na uthsahi bhavathi
One who gets it desires not, worries not, hates not , enjoys not, and does not get excited.
6.Yat gnathwa matho bhavathi, sthabhdho bhavathi, athmaramo bhavathi.
One who attains it becomes intoxicated (be in stupor), still and drunk with self(soul)
7.Saa na kamaya mana , nirodha roopathwad
It is not desire generating as it is by nature a form of renunciation.
8.Nirodasthu loka veda vyapara nyasa
Giving away the social and Vedic customs including religious activities is termed as a form of this renunciation.
9.Tasmin ananyatha that, virodhi shoothaaseena tha cha.
To be one with Him and complete indifference in things which are contrary to him is termed as a form of renunciation.
10.Anyasrayanaan tyage ananyatha.
To sacrifice dependence on any thing but Him is exclusiveness(that which does not have any other thing but Him)
11.Loke vedeshu thatha anukoolasaranam that, virodhi shoothassenatha.
In ordinary life and in Vedic percepts, to do that which is beneficial to Him is exclusiveness(that which does not have any other thing but Him)
12.Bhavathu nischaya dartyathurthvam sasthra rakshanam.
After attaining this state rock stable firmness in Vedic injunctions follow,
13.Anyadha padithya sankhaya
If you do not do this , there is a doubt of a possibility of degradation.
14.Lokopi thava deva kinthu bhojanani vyaparasthva sareera daranavathi
Obey the ways of the world till you attain renunciation, do activities like eating which should continue till the body exists.
15.Tal lakshanathi vachyanthe nana madha bedanath.
The differing definitions of Bhakthi (:devotion”) as viewed by many are listed.
16.Poojadhishwa anuraga ithi Parasara rya.
Intense desire to Worship Him with loving affection , is “devotion” according to Sage Parasara.
17.Kadhadhi shweethi Garga
Intense desire to hear his stories is “devotion” according to Sage Garga.
18.Athma rathya virodhe nethi Sandilya
“Devotion” should be done without conflict to the happiness of the soul, according to sage Sandilya.
19.Naradasthu thadarpithagi, lacharatha that, vismarane parama vyakulathethi.
But according to sage Narada, “devotion” is doing all duties on behalf of Him, and become extremely sad even if you happen to forget Him for a second.
20.Asthy evam evam
This “devotion” is like each of the above views.
21.Yada vraja gopikanam.
This can be best illustrated by the “devotion” of the Gopis of Gokula.
22.Thathrapi tha mahatmyam jnana vismruthya bhavaatha.
In this case at no stage His greatness is ever forgotten.
23.Thad viheenam Jaranam iva
For any “devotion” forgetting His greatness is like an illicit love affair.
24,Nasthyeva tasmin that sukha sukhi twam.
All actions done as a result of this “devotion”(love) is to offer and enhance His pleasure
All actions of this “devotion” are meant for pleasing Him only.
25.Saa thu karma jnana yogebhyo apyadhigathara.
This “devotion” is far greater than the path of action (karma), the path of knowledge (gyana), and the path of disciplined contemplation (yoga).
Because this “devotion” is like the fruit attained by any of these methods..
27.Iswarasyapyabhi mana dweshitwath dainya priyathwacha
Because God also does not like pride and likes humility, “devotion” as a method is better than Karma yoga and Gnana yoga
28.Tasya gnana meva Sadana mithyeke
Some believe that knowledge alone is the path of attainment.
Some others believe that Knowledge and devotion are interdependent on each other
30.Swayam phala roopathethi Brahma kumara
The son of Brahma (sage Narada) believes that “devotion” is its own result
31.Rajagruha Bhojanaadhishu tadaiva drushtatwad
It is just like glancing at a palace or being informed of sumptuous food .
32. Naa thena Raja paridosha kshudha shanthirva
Glancing at the palace does not make the king happy nor hearing of sumptuous food remove his hunger
.Similarly knowing about “devotion”, does not help you at all. You should have “devotion”.
33. Tasmad saiva grahya mumukshubhi
So all those who want to get out of the chain of births and rebirths should only practice “devotion”
34.Tasya sadhanani gayanthy acharya
The learned ones therefore sing about the results of “devotion”
35.Thathu Vishaya tyagath, Sanga thyagath chcha
This “devotion “ reaches completion only when sacrifice is made of pleasurable things and mundane social contacts.
It reaches completion by forever ceaseless chanting
37.Lokebhi Bhagawath guna sravana keerthanad
It also reaches completion by hearing always or singing about the qualities of God.
38.Mukhythasthu Mahad krapayaiva Bhagawat krupa leasad Vaa
But most importantly it reaches completion only by the grace of Great learned people and by the divine grace.
39. Mahad sangasthu durlabhe , agamyo amogascha
The company or the grace of great learned people is difficult to get and is not easily attained and never goes waste.
40.Labhyadhobhi that krupayaiva
The company of great learned people is got only through the grace of God.
41.Tasmim thajjane bhedha bhavath
There is no difference whatsoever between God and his people(devotees)
42.Thadeva sadyatham Thadeva sadyatham
So search for the company of great learned people, for that can be accomplished by that method only.
43.Dussanga sarvathaiva thyajya
Also forever avoid the company of bad people.
44.Kama , Krodha, moha , smrithibramsa , buddhi nasa, sarva nasa karanathwaad.
Because such a company(of bad people) leads to passion, anger, dullness, memory loss loss of wisdom and thus leads to utter ruin.
45. Tharangayidha abhi me sangath samudrayanthi
These (passion, anger etc) though they initially appear in bad company as small waves gather momentum and become like sea.
46.Kastharathi kastharathi Mayam? Ya, sangam sthyajathi yo mahanubhavam sevathe , nirmalo bhavathi.
Who can cross? Who can cross the veil of illusion?
He who is devoid of any worldly attachment serves wise people and does not have a feeling of “mine” can only cross the veil of illusion.
47.Yo viviktha sthanam sevathe, Yo loka bandham unmolayathi, nistraigunyo bhavathi, yoga kshemam tyajathi.
He who lives in a holy place in seclusion, he who cuts of the fetters of worldly attachment, he who goes beyond all characterizations and he who forsakes pleasure as well as yoga only can only cross the veil of illusion.
[Seclusion -- in seclusion from bad association. Renounces requirements and comfort -- becomes utterly dependent on the Lord for everything, not aspiring for them independently )
48.Ya karma phalam tyajathi, karmani sanyasyathi, thado nirdwando bhavathi.
He who sacrifices the result of actions, He who detaches himself from actions and he who sacrifices the two opposites like heat-cold, pleasure-suffering, recognition-insult etc. can only cross the veil of illusion..
49.Vedanabhi sannyasyathi, kevalam vichinna anuragam labhade
He who forsakes even Vedas and reaches the limitless love of “devotion” can only cross the veil of illusion.
50.Sa tharathi, sa tharathi sa lokaam stharayathi
He crosses, He only crosses, and he helps other people also to cross.. the veil of Maya.
51.Anirvachaneeyam prema swaroopam
This extreme love of “devotion” is indefinable
52. Mooka swadanavath
It is (extreme love) like a dumb one tasting , for he does not tell what he feels.
53.Prakasathe kwapi pathre.
Extremely rarely some people are able to explain what they feel.
54.Guna rahitham, kamanaa rahitham, prathi kshana vardhamana, avichinnam, sookshma tharam, anubhava roopam.
This love of “devotion” is without any properties, without any desire, increases every second, micro of micros and it can be described only by really experiencing it.
55. Thad prapya thadevaavalokayathi, thadeva srunothi, thadeva bhashayathi, thadeva chinthayadhi.
One who possesses this love of “devotion”, is able to only see Him everywhere, is able to only hear of Him always, is able to only talk about Him always and always thinks about iHim .
56.Gowni thridha guna bheda arthadhi bhedathwa.
The “devotion” is of three types depending on the type of people viz.Sathwa (pure hearted), Rajasa(warrior in outlook) and Tamasa(debased)
57.Utharasmathu , utharasmath poorva, poorvaasrayeya bhavathi.
Each of the above three categories are better than that coming next , in other words, Sathvika Bhakthi is better than Rajasic which is better than thamasic,
Bhagavad-gItA 7.16: the inquisitive and the man of wisdom, one who is desirous of wealth and one who suffers.
58.Anyasmath saulabhyam bhakthou
This path of “devotion” is the easiest among the paths to attain God.
59.Pramana antharsyam apekshatwath swayam pramanathwath.
This path of “devotion” is its own proof and does not need any other proof.
60.Santhi roopaath, paramananda roopascha
It is personification of peace and ultimate happiness.
61.Lokhanau chintha na karya nivedhi, tha athma loka vedatwad.
One who practices “devotion” should not get worried about worldly affairs, for he has given away all his actions to God.
62.Na thada siddhou , loka vivahoro heya , kinthu phala thyagas that sadanam karya meva
Upon attaining the goal, a devotee should not keep away from worldly actions but perform them without bothering about the result.
63. Sthreethana nasthika vairi charithram na sravaneeyam
Do not hear stories, gossip or news about women, atheist and enemies.
64.Abhimana Dambadhikam tyagyam
Sacrifice egoism and self praise
65.Thadrpidago laachara san kala krodhabhi manadhikam tasminneva karaneeyam
If after offering all actions to god, if feelings of anger, pride and passion come again, they also should be offered to Him.
66.Tri roopa bhanga poorvakam nithya dasa, nithya kanthapa janathmakam va premaiva karyam, premaiva karyam,
Breaking the three distinctions pointed out, one should always serve Him like a servant or wife with unblemished love and definitely with unblemished love.
67.Bhaktha ekandino mukhya
Those devotes only are great who keep Him as their one and only one aim.
68.Kantavarodha romanchasrubhi parasparam labhamana , bhavayanthi kulani prithweem.
These devotees with tear filled eyes, with stuttering voice, and hair standing out discuss always about Him and are a gift to the their clan and to the world.
69.Theerthi kurvanthi, theerthani sukarmi kuvanthi, karmani sachasthri kurvanthi
These devotees make sacred waters more sacred , actions into good actions and holy scriptures more holy.
For they are filled with God.
71.Modanthe pitharo ,nruthyanthi devatha sanadha cheyam bhoorbhavathi.
Because of them the manes rejoice, gods dance and world gets a good leader
72.Nasthi theshu jathi vidhya roopa kula dhana kriyadhi bedha
Within them there is no difference by caste, by knowledge, by looks, by birth, , by wealth or by actions.
For all of them are His.
74.Vadho naa avalambham
These devotees should not enter into arguments.
75.Bahulya avakasada niyathathwacha
With differing opinions expressed nothing can be concluded in these arguments
76.Bhakthi sasthrani mananeeyani thadudbodha sukarmanyapi karaneeyani.
They should always read or hear about “devotion” and they should engage themselves in acts to increase such “devotion”.
77.Sukha dukhe icha labhadhi tyakthe kale, pratheekshyamane kshanartham api vyardhanam na neyam.
They should await the time when happiness, sorrow, desire and profit can be completely forsaken, and keep on thinking about him every second with love.
78.Ahimsa Sathya sowcha, daya asthhikyadhi charithryani parpalaneeyani
They should observe strictly non-violence, truth. Cleanliness, compassion and faith.
79.Sarvada sarve bhavena nischindhidair bhagawaneva bhajaneeya
Always and under all circumstances. Without diverting their mind to other thoughts, they should pray God.
80.Sa keerthayamana seegram eva avirbhavathi anubhavayathi cha bhakthan.
Being sung in such a way by the devotees, He appears before them and makes them realize.
81.Trisathyasya bhakthireva gareeyasi , bhakthireva gareeyasi.
To the one who is truthful to Him in mind, body and words Devotion is the greatest path , It is the greatest.
82.Guna mahathmyasakthi. Roopasakthi, poojasakthi, smaranasakthi, dasyasakthi., sakhyasakthi, kanthasakthi, vatsalya sakthya athma nivedanasakthithanmayadhasakthi, paramavirahasakthi, roopa ekathabhya ekatha satha bhavathi.
Though “devotion” is one quality it is expressed in eleven different ways depending on the individual soul viz 1.longing to hear about His greatness, 2, Longing to see His great form, 3.Longing to worship Him always 4. Longing to always remember Him 5. Longing to forever serve Him as a slave 6. Longing to be His most intimate friend 7. Longing to be his consort 8.Longing to be affectionate to Him like a parent. 9.Longing to sacrifice our soul to Him. 10. Longing to become Himself and 11, Suffering his absence even for a second like a lover.
83.Ithyevam vadanthi jana jalpa nirbhaya eka matha , kumara Vyasa, shukha, Sandilya, Garga, Vishnu, Koundinya, Sesho uddhavarunee bhali, hanumath vibeeshanadha yo Bhakthacharya
Very great devotees like Vyasa, Shukha, Sandilya, , Garga, Vishnu, Koundinya, Sesha, Udhawa, aaruni, Bali , Hanuman and Vibeeshana tell in unanimity without bothering about different opinions expressed that the path of “devotion” is the greatest.
84.Idham Narada proktham Shiva anusasanam Viswaseethi sradhathe sa preshtam labhadha ithi.
One who with utmost faith and belief understands the above exposition of Narada would attain God, would definitely attain God.
OM THAT SATH
 Bhakthi is a Sanskrit word which cannot be explained by one English word. But for sake of clarity I will be using “devotion” instead of the word Bhakthi. By which I mean a devotion of a very special kind.