From Webster's Revised Unabridged Dictionary (1913)
Affect: 1. To act upon; to produce an effect or change upon.
2. To influence or move, as the feelings or passions; to touch.
3. To love; to regard with affection. [Obs.]
4. To show a fondness for; to like to use or practice; to choose; hence, to frequent habitually. 5. To dispose or incline.
6. To aim at; to aspire; to covet. [Obs.]
7. To tend to by affinity or disposition.
Affect: Affection; inclination; passion; feeling; disposition.
Affect: The emotional complex associated with an idea or mental state. In hysteria, the affect is sometimes entirely dissociated, sometimes transferred to another than the original idea.
Effect: 1. Execution; performance; realization; operation; as, the law goes into effect in May.
2. Manifestation; expression; sign.
3. In general: That which is produced by an agent or cause; the event which follows immediately from an antecedent, called the cause; result; consequence; outcome; fruit; as,
4. Impression left on the mind; sensation produced.
5. Power to produce results; efficiency; force; importance; account; as, to speak with effect.
6. Consequence intended; purpose; meaning; general intent; -- with to.
7. The purport; the sum and substance. ``The effect of his intent.'' --Chaucer.
8. Reality; actual meaning; fact, as distinguished from mere appearance.
9. pl. Goods; movables; personal estate; -- sometimes used to embrace real as well as personal property; as, the people escaped from the town with their effects.
{For effect}, for an exaggerated impression or excitement.
{In effect}, in fact; in substance. See 8, above.
{To give effect to}, to make valid; to carry out in practice; to push to its results.
{To take effect}, to become operative, to accomplish aims. --Shak.
Effect: 1. To produce, as a cause or agent; to cause to be.
2. To bring to pass; to execute; to enforce; to achieve; to accomplish.
This was a question posed to Merriam-Webster's site (www.m-w.com)
Q. What is the difference between affect and effect?
A. Read on...
As a verb, to affect means "to act upon or have an influence on": "Sunless days affect my mood." It can also mean "to make a show of; to put on a pretense of; to feign; to assume"; as, "to affect ignorance."
To effect means "to bring about or create"; as, "to effect a change." If you affect something, you do to it. If you effect something, you cause it to be.
Advertising might affect the sales of widgets (by causing them to increase), or it can effect sales (bring them about) if there are no sales at all to begin with. As a noun, effect means "result; consequence; outcome." An effect is that which is produced when you affect something: "The poem affected me deeply; it really had an effect on me."
Affect as a noun is a term from the field of psychotherapy meaning "the emotional complex associated with an idea or mental state." If you are not a psychiatrist or social scientist, you will likely have little use for it. To make it simple, or at least less complicated, keep in mind that usually if you want a noun, the word you want is effect, but if you want a verb, the word you want is affect.