An easily confused pair. Affect with an a is usually a verb; effect with an e is (usually) a noun. When you affect something, you have an effect on it. The usual adjective is effective, which means "having the right effect," or "getting the job done" — an effective medicine, for instance. (It can also mean "in effect," as in "the new policy is effective immediately.")
If the usuals leave you curious, here's the rest of the story: affective as an adjective means "relating to or arousing an emotional reaction"; effect as a verb means "to bring about" or "to accomplish," as in "to effect a change." There's also the noun affect, usually used in psychology, meaning "an emotion" or "feeling." (Lynch)
