Writing Well for the Web: Overcoming the Most Common Mistakes - webreference.com: "They're vs. Their vs. There. They're is a contraction for they are. Their is a possessive pronoun meaning belonging to them or of them. There is the partner of here (which is neither here nor there).
A Test: If you can replace they're/their/there with they are, then they're is correct. If you can replace they're/their/there with his or her, then their is correct. If you can replace they're/their/there with here, then there is correct."