- Affect (transitive verb)
- to cause an effect upon
- Effect (transitive verb)
- to bring into being, to cause
- Effect (noun)
- something that inevitably follows an antecedent (as a cause or agent)
To affect something is to cause it to change in some way. To effect something it to cause it to come into being. Effecting a change affects the changed thing with an effect. What could be more clear?
From Strunk, William, Jr. and White, The Elements of Style, V. WORDS AND EXPRESSIONS COMMONLY MISUSED:
Effect. As noun, means result; as verb, means to bring about, accomplish (not to be confused with affect, which means "to influence").
As noun, often loosely used in perfunctory writing about fashions, music, painting, and other arts: "an Oriental effect;" "effects in pale green;" "very delicate effects;" "broad effects;" "subtle effects;" "a charming effect was produced by." The writer who has a definite meaning to express will not take refuge in such vagueness.
-- StevenNewton
Affect is also used as a noun in psychology, meaning, roughly, "reaction". "The patient received the bad news without affect." "Flat affect" and "neutral affect" are fancy ways of saying "emotionless."