A classic problem of English usage. Unfortunately, the commonest advice usually simplifies the matter as follows:

Affect is a verb meaning “to change or to make a difference”; effect is a noun meaning “the result of a change.”

In fact, both affect and effect can be either a verb or a noun, and there is a much wider range of meaning than indicated above.

Affect, v.

to change, to make a difference, to have a material effect

  • The weather will surely affect my plans this weekend.
  • The hole in the bucket deleteriously affects its utility.

to impress, to touch, to move, to have an emotional effect

  • His severe mien affected my perception of his character.
  • The soprano’s performance affected my mood for a week.

to pretend, to feign, to assume a display for appearance’s sake, to prefer (for appearance’s sake)

  • He affected surprise, even though he had learned of the news beforehand.
  • So persuasively did she affect a certain country folksiness that no one ever did guess that she was a multimillionaire manufacturing magnate.
  • Punk rockers will often affect a countercultural attitude so as to appear authentic to their fans, whereas the production and sale of their products belie such pretensions.

Affect, n.

a disposition, feeling, temper, affection

  • A sombre affect pervaded the funeral parlor.
  • The spring rain suggested to the poet an affect for her verse.
  • His affect was phlegmatic and elusive.

an affectation (rarer)

  • She assumed an affect of indifference [to avoid showing how much she really cared].

Effect, v.

to cause, to bring about, to accomplish or to obtain an effect, to bring to completion [perfective]

  • The signing of the law effected the long-awaited policy.
  • By no means can impossible goals be effected.

to cause, to carry out, to put into effect, to inaugurate [progressive]

  • One must pay the insurance premium to effect coverage.
  • Step by step, we can effect the changes we seek.

Effect, n.

a state or fact of being operative or in force

  • Although no longer in effect, the law still holds sway in the civic consciousness.
  • This toilet is out of order with immediate effect.
  • The insurance certificate takes effect on the first of the month.

a result, a consequence, a thing accomplished, a phenomenon

  • Although the law has been repealed, it still retains an effect in the civic consciousness.
  • Wind is an effect of the mixing of air masses of different temperatures.
  • The team hopes that the effect of their rigorous training shall be victory and glory.
  • The Pied Piper effect suggests that if we can persuade the chair, the committee will follow.

a (desirable) purpose, a (pleasing or remarkable) gestalt [related to previous sense]; often pl.

  • Mozart deploys the trombones to great effect in his Requiem.
  • Confetti at a party often has a disappointing effect.
  • The impressionistic color effects of painters like Monet helped give their movement its name.