The word "you're" is a contraction of "you" and "are". Smash them together until you get the apostrophe. "you are"; "youare"; "you're".
The word "your" is a possessive pronoun, meaning that the object in question belongs to you. Read the post on "it's" and "its" for a more detailed explanation. This is exactly the same mistake, except for the identity of the owner of the property in question.
Remember the difference by trading out the "you're / your" for the words "you are". If you're the owner of your book, you would not say, "You are the owner of you are book." The first "you are" makes sense, the second one doesn't; therefore you use the contraction in the first spot, and the possessive in the second spot. "You're the owner of your book."