* Hers: If you're talking about possession, "hers" is the opposite of "his" referring to something belonging to a female.
* Her: If you're referring to a female pronoun, "her" is the opposite of "his".
* Their: If you're talking about something belonging to a group of people, "their" is the opposite of "his" (and "hers").
Example:
* His book is on the table.
* Hers is on the shelf.
* Her name is Sarah.
* Their dog is very friendly.