Affect is usually a verb meaning "to influence." Effect is usually a noun meaning "result." When used as a verb, effect means "to cause."
NOTE: If you're in a professional field related to psychology or psychiatry, you are probably familiar with a special use of affect (pronounced with the accent on the first syllable) as a noun meaning "an expressed or observed emotional response." However, this technical term seldom appears in everyday (non-technical) writing.
Examples:
Peace is a natural effect of trade. War affects trade in various ways.
Corrections:
"In a table entitled Excessive vitamin intake, which accompanied the article, Watchdogs to set limits on vitamin intake (page 7, August 30), it was said that boron 'might effect reproduction.' No such miracle supplement is yet on the market. 'Affect' was the intended word."
(Corrections and Clarifications, The Guardian, September 3, 2002)
Practice:
(a) Scientists continue to study the _____ of sweeteners on humans.
(b) Controversies over sweeteners have not seriously _____ the sales of diet soft drinks.
