From Grammar Gremlins, Knoxville News Sentinel 8/9/09
by Don K. Ferguson
The misuse of "your" for "you're" is far too common.
I have seen this error several times recently, mainly in e-mail messages.
It is common enough that several handbooks make note of it. One says the two words are confused surprisingly often.
"Your" is the possessive of the pronoun "you."
"You're" is a contraction of "you are."
Here are examples from the Gregg Reference Manual that show both uses:
1. Your thinking is sound, but we lack the funds to underwrite your proposal.
2. You're thinking of applying for a transfer, I understand.