One is most commonly used as a verb, the other as a noun. Do you know the difference? Check out this brief slideshow or, for more information on how and when to use affect and effect, read the YourDictionary article "Affect vs. Effect Grammar Rules."
Example: The dog's death will really affect his owners.
Although there is a noun form, it is rarely used. It is a psychological term meaning emotion (usually shown by facial expression).
Example: Her cold stare and blank affect was hard to read.
Example: The new stop sign had a good effect on the speed of the motorists I saw.
You will sometimes see it used as a verb meaning to make happen or implement something.
Example: The best way to effect change in your community is to volunteer.
For more information on how and when to use affect and effect, check out the YourDictionary article Affect vs. Effect Grammar Rules.