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.
edit 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.
edit Examples of Affect vs. Effect
- Carbon dioxide emissions affect the environment.
- The effect 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.
- Reducing the marketing budget will affect sales growth.
- We know the new marketing campaign is really effective because sales are up 350%.
- Do you know the side effects 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.
edit Usage in Special Cases
Like many other words in the English language, affect and effect have more than one meaning.
edit 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.
edit 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".
edit 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.