The following explanation was sourced from http://www.wisegeek.com/what-is-the-difference-between-affect-and-effect.htm:

The word affect is primarily used as a verb in English. It has two main meanings, which are closely related. Affect may mean to alter the feelings of, or to change the mental state of someone or something. It may also mean, in a broader sense, to change or affect someone or something in any way. So it is that we might say: How will the election affect the course of history? We might also say: The painting affected him so deeply he could not speak.

The word effect, in contrast, is used primarily as a noun in English. It has a number of related meanings, but generally speaking refers to the result of something, the power something might have to get that result, or a phenomenon in the world. An example of the word used to refer to the result of something would be: The election had no effect on the course of history. We might also say: The effect of the painting on him was profound. Phenomena are often also referred to as effects, such as the photoelectric effect or the greenhouse effect.

Affect may rarely be used as a noun, but this usage is esoteric and somewhat archaic. In psychology one may speak of a person’s affect, which refers to their mood or mental state. So we might say: The patient exhibited a flat affect, responding to no stimuli.

Effect may also be occasionally used as a verb. When it is used in this way, it refers to something having a direct effect, or making it happen. It is usually suffixed by -ed, and takes an object. An example of this usage would be: The election at last effected the change the people had been hoping for.