It seems quite a few people have trouble with this one. Let me simplify:
Affect: verb. Effect: noun.
Chances are that's all you'll ever need to know. Affect can also be a noun—an emotion or desire, especially relating to influencing behavior— but mostly that definition pertains to psychology. For everyday writing, the above statement is all you need.
Here's a nice acronym to help you remember:
SHAVEN: S'not Hard: Affect Verb, Effect Noun.
There you have it. If you get confused, think about SHAVEN.
Also, you know, forget about the noun version of affect. You're probably not a psychologist, right?