Usage tip: affect vs. effect

In nearly all cases, affect is a verb (meaning "to have an influence on"), and effect is a noun (meaning "result" or "influence"). Thus:

CORRECT
That movie had a strong effect on me.
The news report affected me greatly.
How does gravity affect matter?
What are the effects of solar radiation?

INCORRECT
The book I just read really effected me.
I don't drink because I don't like the affects.
Can you describe some of the affects of commercials?
Will Internet usage effect how we learn information?

One way I often remember it is this: Just like a comes before e in the alphabet, you have to affect something before you see an effect.

Just to confuse things, of course, there are situations in which affect is a noun, and in which effect is a verb. However, those usages are not parallel like the ones above.

Affect as a noun is almost exclusively used in psychology (and by people who are not in psychology but who want to sound like they are). In that context, affect refers to a person's behavior, mood, and general emotional state. (Also in this context, the word is pronounce "A-fekt", not "a-FEKT".)

Effect as a verb means "to put into motion" or "to cause". In this context, the word is pronounced "ee-FEKT", as opposed to the more common "uh-FEKT". It's almost always (at least in my experience) used with "change," as in "to effect a change". Again, this is an uncommon usage.

In general, to figure out whether to use affect or effect, try replacing the word with another noun or verb. If you can replace it with a verb, use affect. If you can replace it with a noun, use effect. But please, whatever you do, do not use impact!!