The most common usage of affect is as a verb, meaning "to impact". Effect is usually a noun, meaning the result or consequence of something. However, both affect and effect have alternate meanings when used as different parts of speech.

Differences in Usage

In general, affect is a verb and effect is a noun. When X "affects" Y, it is said that X produces an "effect" on Y. In passive form, Y is "affected" by X.

Examples of Affect vs. Effect

Carbon dioxide emissions affect the environment.

the environment. The effect of global warming is that glaciers will melt and sea levels will rise.

of global warming is that glaciers will melt and sea levels will rise. The results of the 2008 presidential election in the United States were likely affected by the war in Iraq.

by the war in Iraq. Reducing the marketing budget will affect sales growth.

sales growth. We know the new marketing campaign is really effective because sales are up 350%.

because sales are up 350%. Do you know the side effects of Tylenol?

of Tylenol? Doctors must keep in mind that X-rays in CAT Scans may affect the body adversely with repeated exposure.

As a rule of thumb that works in most common usage scenarios, it is advisable to use affect as a verb (transitive, intransitive, active, passive) and to use effect as a noun.

Usage in Special Cases

Like many other words in the English language, affect and effect have more than one meaning.

Affect as a Noun

When used as a noun, "affect" refers to the external display of one's emotions or moods. The pronunciation of the word "affect" is slightly different when used as a noun. It is pronounced with a short A, as in apple, rather than the "uh" sound found in amount and affect when used as a verb. For example, consider the sentence John was unaffected by the impassioned performance. This means John did not display any emotions.

Effect as a Verb

When used as a verb, "effect" means to execute, produce, or accomplish something. It is pronounced the same way whether used as a verb or a noun. For example: The best way to effect change in society is to work at the grassroots level. Here "to effect change" means "to produce change".

Effects as a plural noun

The word "effects" can also be used to mean "a person's belongings". For example, He left without any of his personal effects.