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This is a tricky one as both affect and effect can be used both as a noun and as a verb. Usually, however, if a verb is being used then affect is correct and if a noun is being used effect is correct.

Affect
As a verb, affect means to influence or to adopt a pose or manner.

The outcome of treatment is affected by patients' degree of compliance.

The girl affected a couldn't care less attitude.

As a noun affect means an emotional state. It is rarely encountered outside the realm of psychiatry. So, most people are unlikely ever to use affect as a noun; however, you may well come across it in medical titles.

Effect
We don't think anyone is likely to have difficulty in comprehending the usual meaning of effect: something brought about; a result.

The effect of treatment was an improvement in patients' quality of life.

But effect may also be used as a verb, meaning to cause to happen.

The drugs effected a significant improvement in patients' quality of life.