what is the difference between the word "my" and "mine" ?

 "My" is an adjective. 
This is my friend. 

This is your, his, her, its, our, your, their friend. 

This is my book 


"Mine" is a possessive pronoun. 
This is a friend of mine. = This is my friend. 

This is a friend of mine, of yours, of his, of hers, of ours, of yours,of theirs. 

This book is mine. (yours, his, hers, ours yours, theirs) 

His cat is black. Mine is white.