What is the difference between 'affect' and 'effect'?

This is one of the most commonly confused word pairs in English, but a simple test helps: if you're describing an action (a verb), you almost always want "affect." If you're naming a result (a noun), you almost always want "effect."

Examples

• "The rain will affect our plans." (verb — it influences the plans)
• "The rain had a big effect on our plans." (noun — the result of the rain)

The less common exceptions

"Effect" can occasionally be a verb meaning "to bring about" (as in "effect change"), and "affect" can occasionally be a noun in psychology, referring to an observable emotional expression. These uses are rare in everyday writing — for nearly all common sentences, the verb/noun rule above holds.