Affect is usually a verb. It means to have some kind of effect or impact on someone. To affect people is to influence them, often on an emotional level.
The lousy weather affected Cindy’s mood.
We were greatly affected by the sadness in her voice.
In some rare instances, affect can be a noun. It is a technical term in psychology, and more generally it refers to the outward expression of emotion (especially on one’s face). However, you might be better off using the word affectation if you mean to describe an expression of feeling that is not genuinely felt.
Most of the time effect is a noun. It refers to the result or outcome of some kind of action.
The rise in interest rates had a negative effect on the housing market.
One effect of the new law has been an increase in jaywalking.
Occasionally effect can be used to as a verb, to mean make, cause, achieve, or create. It is often used in the expression “to effect change,” a rather hackneyed phrase.
If you want to effect change, join the police force!
The main thing to remember is that affect is a verb meaning to influence or have an impact, and effect is a noun that refers to the result of an action. In other words, whenever you affect people you have an effect on them.
There are, however, some exceptions to these rules. Sometimes effect can be used as a verb to mean make or cause. In that case it should have a direct object (e.g., He effected a change of policy). Conversely, affect can be used as a noun to refer to a feeling or emotion expressed outwardly.