Sometimes you will see the word your’s written as the possessive form of the word your. It is incorrect.
According to http://www.elearnenglishlanguage.com:
The idea that yours needs an apostrophe comes out of the fact that on virtually every other word, ‘s indicates possession, so English speakers sometimes think yours should be spelled your’s. However, this is always incorrect – yours is the only correct spelling.
Fay Moore: I Want To Be a Writer 