Now, I'll be the first to admit that I'm not perfect, and neither is my writing. I do pride myself, however, on my ability to differentiate between phonetically similar, yet definition-ally different words(homonyms).
By far, the most glaringly obvious example of this would be the difference between "your" and "you're". So many people I know think that these words are interchangeable. This is not the case. They are two completely different words with two completely different definitions.
Definitions of "your" and "you're", respectively, from dictionary.com:
Your - [pronoun] -1. a form of the possessive case of you used as an attributive adjective: Your jacket is in that closet. I like your idea. Compare: Yours
2. one's, used to indicate that one belonging to oneself or to any person: The consulate is your best source of information. As you go down the hill, the library is on your left.
3. used informally to indicate all members of a group, occupation, etc., or things of a particular type: Take your factory worker, for instance. Your power brakes don't need that much servicing.
You're - contraction of you are: You're certain that's right?
As you can plainly see, these words DO NOT mean the same thing. In fact, the second is not even a singular word, but a shortened form of two different words, you and are, as in You are using the word "you're" incorrectly.
When in doubt, say the sentence you are trying to convey out loud to yourself. Can you replace the word "your" with "You are"? Is the subject of your sentence DOING something? If so, use "you're", or, even better, just use "You are" to eliminate any possible confusion.
Does the subject of the sentence BELONG to someone? Then use "your", as in Your status is so funny.
In fact, when you say "your" and "you're" out loud, they should sound different. "Your" should be pronounced "yore", as in "the saintly days of yore". "You're" should sound more like "yew-er".
This concludes today's lesson on grammar. In the words of a good friend of mine, "You're right, people are stupid though, you're smart. Now go get your hair did."