This is a tough one for many people, including some professional writers. Generally, affect is a verb and effect is a noun. When you affect something, you have an effect on it. The mistake most often made is writing affect instead of effect. Stick with me here and refer back when needed -- once you set your mind to it, you'll see it's really not too hard.
Affect means to act upon or to move -- the photos affected me so much I cried. It also can mean to pretend -- on her first day of high school, she affected a calm she did not feel.
An effect is a result, a consequence. The well-written blog had quite an effect on me.
More examples of the correct use of effect: Movies are full of sound effects and other special effects. Your house is full of your personal effects. When things become effective, they go into effect or they take effect. I strongly disagreed and wrote her to that effect. On Halloween, I turned down the lights and put on spooky music, but it was all for effect. Cause and effect.
Of course, to throw a monkey wrench into the works, effect can (rarely) be a verb, most often when talking about change -- President Obama's administration seems to be finding it tough to effect true change. In this sense, it means to make happen, to create.
Psychologists may speak of a person's affect, with the accent on the first syllable (AFF-ect). This refers to a person's emotional response. I've never used it this way in my lifetime, and I'm guessing you probably won't either.
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