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There, their and they're

The longest of these (they're) is the easiest to not get wrong. The apostrophe indicates that a letter is missing, in this case a:

They're = they are
e.g. They're going to the club.

Their is a possessive word that shows ownership of something.
e.g. Their house isn't as big as ours.

There is a word with many uses, indicating a place or position.
e.g. My new Honda Jazz is over there. There is a cat sat on its roof.
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Your and you're

This is one of the most common confusions. The problem here is similar to that of there and they're, in that you're is also a contraction.

You're is really you are, the apostrophe indicating that the a is missing:
e.g. "You're really tall and your spiky hair makes you seem taller!"

Your is a possessive pronoun (used with hair above)
e.g. your car, your phone, etc.

One common misuse of the word your is in questions such as:
"Hope your okay."

It should, of course, be "Hope you're okay."

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