Your vs. You're

"Your" is a possessive pronoun, meaning belonging to you (see also the entry for yours).  An example of proper usage is: "Is that your bicycle?"  When intending to convey "you are," the word that should be used is "you're."  "You're" is the contraction for "you are."  The words "your" and "you're" are often mixed up in informal writing, mainly by using the former in place of the latter (although I've also seen the error made in reverse).

Incorrect: "Your the best."

Correct: "You're the best."

Correct: "You're taking your bicycle with you, aren't you?"

Summary: If you mean "you are," use "you're."  If not, "your" is the spelling that should be used.