your and you're

Your and you're (much like its and it's) are two different words with different meanings. Your is the possessive form of "you." (For example, Is this your cat?) You're is the contraction for "you are." (For example, You're going to pick up cat food later, right?)

No matter what some folks say, I maintain that "ur" is not a word. Arguably, it's an acceptable shortened form of "your" if you happen to be writing a text message and can't spare the extra two letters. It should not, however, find its way into any kind of legitimate communication, like an e-mail to your boss, for example.