It's the "W" or something that makes them easily confused. Anyway, the short story is a W-4 is what you fill out when you start a job so that your employer knows how much to withhold for you. A W-2 is the form you get at the end of the year that is used to report taxable wages and withheld taxes. By now, you should've received all your W-2's. Check out my previous post "No W-2? No Problem!" if you still haven't received yours and need to know how to file.
Now that you know what the difference is between the two, how do you read those darn things? Sharon over at That Baby Boomer Generation has conveniently answered both these questions already. See her posts on reading your W-2 and reading your W-4.