From the AP Stylebook :

Affect, as a verb, means to influence: The game will affect the standings.

Affect, as a noun, is best avoided. It occasionally is used in psychology to describe an emotion, but there is no need for it in everyday language.

Effect, as a verb, means to cause: He will effect many changes in the company.

Effect, as a noun, means result: : The effect was overwhelming. He miscalculated the effect of his actions. It was a law of little effect.

(I still don’t quite understand.)