Word	Meaning	Example
Affect	To act upon.	DDoS attacks affect web sites in a very bad way.
Effect	The creation of a desired impression.	The effect of a DDoS attack is almost the same as getting listed on Slashdot.

A good way to think about which one to use is to think about cause and effect. If that phrase applies to the point you are trying to get across, you want effect, otherwise you probably want affect.