What is the difference between affect and effect?

In the overwhelming majority of cases, affect is the verb and effect is the noun. Something affects you; the result is an effect.

The delay affected our plans. The delay had an effect on our plans.

A useful check: if you can replace the word with "influence", you want affect. If you can put "an", "the" or "this" in front of it, you want effect. Another mnemonic that survives contact with real sentences is Affect is an Action, Effect is the End result.

Both words do have a second, less common form, which is the entire reason the confusion persists.

Effect works as a verb meaning to bring something about, most often in the fixed phrase to effect change. Note the difference this makes: the new manager affected the process means she influenced it; the new manager effected change means she caused change to happen. The verb form is formal and comparatively rare.

Affect works as a noun in psychology, where it means observable emotional expression — a clinician might write that a patient displayed flat affect. Outside that field you will almost never need it, and it is stressed on the first syllable, unlike the verb.

So the rule of thumb holds nearly always, and the exceptions are narrow enough to learn as exceptions.

One further trap: affective means relating to emotion, while effective means producing a result. They are not variants of each other and are not interchangeable.

Many writers dodge the choice entirely by reaching for impact as a verb — the delay impacted our plans. It is not wrong, but it is heavily overused in business writing and reads as evasive precisely because it avoids committing to a meaning. Affected is usually the better word.