1. Affect is a verb; it means "to have an impact or influence upon."

·         Incorrect: The moon effects the timing of the tides.

·         Correct: The moon affects the timing of the tides.

2. Effect is generally a noun.

·         Incorrect: I have that affect on people.

·         Correct: I have that effect on people.

3. Two exceptions are as follows:

3a. Psychologists sometimes use "affect" (pronounced AFFect, unlike the verb form affECT) to mean a person's emotional presentation. Example:"Her affect was flat."
3b. "Effect" is used as a verb in specific idioms, such as "to effect a change"--that is, to cause a change to occur. Example: "He effected the change in the rules so that he would be the chairperson permanently."

Campbell, M. D. (n.d.). Commonly Confused Words. Retrieved December 17, 2012, from http://public.wsu.edu/~campbelld/keyto.htm