What?
A lot of words in the English language (and other languages I'm sure) sound the same but have different spellings and/or different meanings. The problem is, a lot of people never learn these different spellings and meanings and just go about their merry way using the wrong words. When somebody is speaking, you can't tell. However, once you start writing stuff, like web pages or email, you make the big mistake of confusing your readers. People that don't already know what the right homonym is will learn it wrong, from your usage. Those that know the right word will just think you're a moron. So what should you do? Learn it right!

There, Their, and They're

There
There is usually used to describe a physical place or location, or at least a noun. It can also be used in other ways, but I don't know how to best describe it. The following examples should help:

• Over there is my brain! I left it on the side of the road.
• There exists a time when every man must learn his homonyms.
• Do you see now that there is a difference?

Their
Their is used to show possesion. Just as 'his' shows possesion of an object by 'him', and 'her' shows possesion of 'her', their shows possesion of an object by a group, they.

• It's their crap, not yours.
• Their idea to teach people homonyms was wonderful.

They're
They're is a contraction of the words "they" and "are", as indicated by its apostrophe. You use it to describe a group of objects or whatnot. If your sentence doesn't sound right by replacing they're with they are, then you're using the wrong form of there/their/they're.

• They're stupid; they don't even know their homonyms.
• It's good that they're reading this page though.

To, Two, Too

To
To is hard to explain. It's kinda like an implied verb prefix. Read the examples.

• To be, or not to be, that is the question.
• I have to go to the bathroom.
• You want to go to the concert with me?

Two
Two is the spelling of the number 2. Use it when referring to 2 elements of something.

• There were two ducks in the pond, not three.
• What do you call a blonde with two brain cells? Pregnant!

Too
Too can mean either 1) also, or 2) an excess of.

• I have to go to the bathroom too. (case 1)
• You go to the bathroom too much. (case 2)
• Hey, I want some beer too! (case 1)
• You are too young to go in the bar! (case 2)

Are, Our

Are
Are is hard to explain also. Read the examples.

• Are you going to the concert?
• There are five ducks swimming around the pond.
• I bet you are tired after that long drive!

Our
Our shows possesion. Use it when you are referring to a group you are in that 'owns' something.

• Hey, that is our money, don't touch!
• Our pineapples are the best in town.
• Would you like to tour our factory?

Your, You're

Your
Your shows possession.

• Is this your money sitting here?
• Can I have some of your Coke?

You're
You're is a contraction for "You Are". If you can't replace you're with you are and have your sentence work, then you've messed up.

• After that run, I bet you're tired as hell.
• Hey baby, you're pretty hot. (don't try this one in real life)

Bonus Section
See if you can identify the correct sentence....

1. Your pretty nice to help us invest are money like that.
2. You're pretty nice two help us invest our money like that.
3. Your pretty nice to help us invest are money like that.
4. You're pretty nice to help us invest our money like that.
5. You're pretty nice too help us invest are money like that.
6. Your pretty nice to help us invest our money like that.