(From Fowler, H. W 1926, A Dictionary of Modern English Usage. Oxford: Oxford University Press.)
These verbs are not synonyms requiring differentiation, but words of totally different meaning, neither of which can ever be substituted for the other. Affect (apart from other senses in which it is not liable to confusion with effect) means have an influence on, produce and effect on, concern, effect a change in: effect means bring about, cause, produce, result in, have a result. These measures chiefly affect the great landowners. It does not affect me. It may seriously affect (i.e. injure) his health. A single glass of brandy may affect (alter for better or worse the prospects of) his recovery. A single glass of brandy may effect (bring about) his recovery. This will not affect (change) his purpose. This will not effect (secure) his purpose.