Possessive pronouns are used in English to avoid repeating information that is already clear.
This book is my book, not your book.
( Sounds repetitive )
This book is mine, not yours.
( Mine and yours are possessive pronouns. )
In the sentence, mine replaces my book and your book is replaced by yours.
The possessive pronouns are:
I - Mine
The car is mine.
You - Yours
Those books are yours.
He - His
That house is his.
She - Hers
The ice cream is hers.
We - Ours
These boxes are ours.
You-all - Yours
The computer is yours
They - Theirs
The bananas are theirs.
* Remember when using possessive pronouns, you don't use apostrophes.
Of Yours
It's very common to say "a friend" or "some friends" + of + possessive pronouns. For example:
I saw a friend of mine today.
She's a friend of yours.
One of your friends like me.
Yours faithfully / Your sincerely
Yours in English is also used with faithfully and sincerely at the end of writing a formal letter or an email. For example:
Yours faithfully
(if it's someone that you don't know.)
Yours sincerely
(if it's someone that you know.)