In most common cases, think of affect as a verb, meaning to change, and effect as a noun, meaning the change. However, as verbs, they are easy to confuse: affect means to make influence on, to move, or to make a difference to; effect means to bring about or cause to happen. Thus Pain affects me, Constant pain may effect behavioral change, and The effect of pain may be sorrow. (The second example above works with either affect or effect: the former implies that behavioral change, which was in effect prior to the constant pain, has been influenced or altered; the latter, effect, indicates that constant pain has brought about behavioral change.) Though uncommon, affect can also be a noun in psychological writing, referring to someone’s emotional state.
Unlike effect, the past participle of affect may also act as an adjective, meaning influenced, artificial, or designed to impress. Thus The hurricane effected architectural improvements and Most of the affected area was destroyed.