(1) Bhaagvat means Related to God. 

(2) Bhaagvat Puraan
There are 18 Puraan, 12 of which contain the life story of Shree Krishn, while 6 do not. One of them is Bhaagvat Puraan which is the main source of Krishn's life. These 12 Puraan are - (1) Agni Puraan, (2) Bhaagvat Puraan, (3) Brahm Puraan, (4) Brahmaand Puraan, (5) Brahmvaivart Puraan, (6) Garud puraan, (7) Koorm Puraan, (8) Ling Puraan, (9) Matsya Puraan, (10) Padm Puraan, (11) Vaayu Puraan, and (12) Vishnu Puraan.

Another book which is not called Puraan, but is identical in style of Puraan and was written around 400 AD, is Harivansh. It also contains the life story of Krishn. Mahaabhaarat continued to be expanded for 1600 years and took its final shape around 250 AD. Most material came from one small Puraan written by Ved Vyaas in 1100 BC. These 18 Puraan, perhaps 99.9% of the material of which does not constitute scripture, came about during 250 AD and 1050 AD.

So one of these 18 Puraan is Shree Mad-Bhaagvat Puraan. It was composed around 850 AD, later than many Puraan. It seems to have taken the best of all the preceding Puraan. It is the most popular and revered of all Puraan. It consists of many mythological stories, 9 incarnations of Vishnu etc. Its 10th chapter is the biggest chapter containing Krishn's story. Because of the story form and the ease of understanding, and because of the extreme difficulty of understanding Ved and Upanishad, Bhaagvat Puraan almost replaced the Ved and Upanishad for the next 1100 years. Many incidents of Krishn's life written in Bhaagvat are not supported by accounts found in older Puraan.

(3) Bhaagvat Dharm
In Bhaagvat Puraan, Skandh 6, Chapter 3, Yam Raaj says to his Paarshad that Bhaagvat Dharm is known only to 12 people - (1) Brahmaa Jee,  (2) Devarshi Naarad,  (3) Bhagavaan Shankar,  (4) Sanat Kumaar,  (5) Kapil Dev,  (6) Swaayambhuv Manu,  (7) Prahlaad,  (8) Janak Jee,  (9) Bheeshm Pitaamaha,  (10) Raajaa Bali, (11) Shuk Dev Jee, and  (12) I (Dharm Raaj).