The words mine, ours, yours, his, hers and theirs are called possessive pronouns.  We use them to say who something belongs to. 

Both possessive determiners and possessive pronouns express a relation, often the fact that someone has something or that something belongs to someone.

A noun follows a possessive determiner or possessive adjective  but NOT a possessive pronoun. 
Examples:
     It’s my book. OR This book is mine.
     These CD-Roms are not mine.  Are they yours, John?
     These shoes are ours and those are yours.

We leave out the noun if it is clear from the context what we mean.  When we do this, we use a possessive pronoun instead of a possessive determiner or possessive adjective.
Example:
     I'll look in my diary.  I haven't got mine with me.