Affect vs. Effect

Affect and effect are two words that are commonly confused.

affect is usually a verb (action) - effect is usually a noun (thing)

Hint: If it's something you're going to do, use "affect." If it's something you've already done, use "effect."
To affect something or someone.

Meaning: to influence, act upon, or change something or someone.

For example: The noise outside affected my performance.

The use of "effect" as a verb is what causes the most confusion between these two words.

To have an effect on something or someone.

!Note: effect is followed by the preposition on and preceded by an article (an, the)

Meaning: to have an impact on something or someone.

For example: His smile had a strange effect on me.

!Effect can also mean "the end result".

For example: The drug has many adverse side effects.

!Note - Just remember: "affect" = to influence, and "effect" = to bring about.

Rules from http://www.oxfordlearnersdictionaries.com/definition/english/affect
affectverb = ‘to have an influence on somebody/something’:Does television affect children’s behaviour? It is not a noun.effectnoun = ‘result, influence’:Does television have an effect on children’s behaviour?effectverb is quite rare and formal and means ‘to achieve or produce’. They hope to effect a reconciliation.

References

http://www.oxfordlearnersdictionaries.com/definition/english/affect
http://www.learnenglish.de/mistakes/affectvseffect.html