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Affect is always a verb.

-> "to influence" or "to produce a change in"

example: Poor diet affects one's health.
            (means, poor diet influences or produces a change in one's health.)
example: The passing of his parents affected him.
            (means, the passing of his parents influenced or produced a change in him.)

Effect is usually a noun.

-> means "the result," "the change," or "the influence"

example: Poor diet has a grave effect on one's health.
            (means, poor diet has a grave influence on one's health.)
example: The passing of his parents had a deep effect on him.
            (means, the passing of his parents had a deep influence on him.)
example: What we are experiencing now are the effects of global warming.
            (means, what we are experiencing now are results of global warming.)

-> Effect is sometimes, though not often, used as a verb, meaning "to cause."

example: The nonprofit organization mobilizes to effect awareness on women empowerment.
            (means, the nonprofit organization mobilizes to cause awareness on women empowerment.)
example: The speaker is very effective in effecting critical thinking amongst the listeners.
            (means, the speaker is very effective in causing critical thinking amongst the listeners.)

Source: Commonly confused words: affect/effect

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