These two words are discussed at length in the Common Grammatical Mistakes section of Grammar.com. Click here for that discussion.
Both words can be used as either nouns or verbs even though many sources will claim that affect is a verb and effect is a noun. But actually, both can act as verbs and as nouns.
As a verb affect means “to influence” while the verb effect means “to produce a result.” The noun phrases “cause and effect” and “special effects” might help you remember that effect usually acts as a noun.
You should use effect as a verb when one thing causes another. To “effect changes” means “to create new changes.” To “affect changes” means “to influence or modify changes already made.”
Affect is not very common as a noun but can be used to describe a feeling or emotion especially as evidenced by facial expression or physical demeanor. As a noun, effect is the result of a change or course of action.
Example: The effect of the new policy to lower wages was obvious from the downcast affect of the office staff. Their attitudes will certainly affect their performance and the overall success of our business. If we fail to show a profit, the board of directors will effect further changes and reduce salaries even more.