Effect

The word effect is often confused with affect.

To quote "Rob, Lyme Regis" from the BBC News Magazine:
 * If you do something to change a situation, then you "effect" a change. If your circumstances are changed by an action, then the change has caused an "effect". You cannot "affect" a change in something, nor can you be "effected" by one.

The correct use of "effect" is usually as a noun, a synonym of result. In a less common use as a verb, it means to create the the result (not just to influence it as "affect").

Samples
Correct:
 * The effect of the rainstorm was an overflowing drain.
 * The political rally was held to effect a change in the laws.

Incorrect:
 * His mood effected me.
 * One of the affects of her better eating habits was weight loss.