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

Pronouns are used instead of nouns.  Note that a noun is a naming word.  We use personal pronouns for the speaker (I) and the person spoken to (you).  We use he, she, it and they to refer to other people and things when it is clear from the context what we mean.