Affect and effect. These innocuous words are too often confused, and they're guaranteed to attract the wrath of the spelling police when they are. Even the most respected and attentive wordsmith has accidentally left an apostrophe in the wrong its; a slip of you're for your will have you written off as a lost cause and hopefully ignored; but muff this one and the affronted grinding of teeth and the hovering of cursors above downvote buttons is nigh audible.

So, I hear in gratifyingly panicked tones, which is which, and which is the right one?

If that was you, then let's start with the simple, oft-shouted answer:

Affect is a verb: "Vodka doesn't affect me."
Effect is a noun: "Vodka has no effect on me."

Affect is what something does to something: when the affecting is done, what's left is the effect. This lends itself to a handy mnemonic:

affect is the act
effect is the result

a, e, get it? Simple.

This is a raw generalization that covers the most frequent uses. As a rule of thumb, it will get you far - you can remember this much and run negligible risk of ever having your work softlinked to "Affect" and "effect" mean different things. Maybe you should look it up. However, as is always the case with English, there are other uses and cases that you will come across and maybe even feel like using. For the advanced student and the wannabe grammar cop, the following is a summary of the whole affect-effect squaredance. The key cases above are emphasized for easy reference.

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affect

• As a verb:

  • Meaning "bring about a change in" - the act of something altering something.

    "Don't let downvotes affect you."
    "It is unknown how gangsta rap affects foetal development."
    "Satellite communications may be affected by sunspot activity."
    

  • Meaning "feign" or "put on" (a voice, character, etc.). The result is an affectation.

    "First, affect the manner of a bonobo in heat..."
    

• As a noun (uncommon!):

  • Meaning "emotional response". Although its original sense is preserved in related words like affection and disaffected, this archaic use of "affect" now survives only in the fields of philosophy and psychology, where it has acquired a more specialized meaning. It should only be invoked if you are a) a philosopher or psychologist addressing another philosopher or psychologist, or b) arguing about the proper uses of the word "affect" and trying to sound clever. If (b), remember to pronounce it with emphasis on the first syllable - AF-fect - and then get a life.

    "Schizophrenics exhibit inconsistent or inappropriate affect."
    

effect

• As a noun:

  • Meaning "change brought about" - the result of something altering something.

    "Don't let downvotes have an effect on you."
    "The effects of gangsta rap on foetal development are unknown."
    "Sunspot activity can have inconvenient effects."
    

• As a verb:

  • Meaning "bring about" - the act of something creating an effect. This is not the same as affect: the object of affect (verb) is the thing being altered, while the object of effect (verb) is the alteration that results. Put another way:

    x affects (verb) y, so
    x effects (verb) an effect (noun) on y.

    Note that this is a well-worn example of verbing weirding language.

    "We are confident these reforms will effect substantial savings."
    

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That's it. Good luck, and if pain persists, see your dictionary.