Although these are two separate words, their similar spellings and senses mean that they are commonly confused. The reason they are so similar is that they share closely related origins – affect is from Latin ad+facere ‘to try to’ and effect is from Latin ex+facere ‘to make happen’. Ever since their first appearance in English in the Middle Ages the spelling of these two words has been confused; this confusion can even be traced back to medieval French, from which the words were borrowed into English. It was only in the 19th century that our standard spelling practices for these words were established. The basic rule today is that effect is the correct spelling of the noun, meaning ‘result’, ‘consequence’, and affect is the correct spelling of the verb, meaning ‘influence, make a difference to’. That’s not quite the whole story though, since Modern English does have a verb to effect, meaning ‘to produce, accomplish, or bring about’, as in ‘effect change’, or ‘effect a reconciliation’, but this word is much less common than the noun. There is also a noun affect, meaning ‘emotion, desire’, but this tends to be restricted to technical contexts. So, if in doubt, you can stick to the general rule that affect is the verb and effect is the noun.
Advice on how to avoid confusing these words is offered by numerous style guides. H.W. Fowler is characteristically brusque in dismissing any difficulty in distinguishing these two words, although I can’t help feeling that his definition of affect would have been clearer if he’d avoided using effect: ‘produce an effect on, concern, effect a change in’. The Guardian Style Guide is more willing to acknowledge the problem, and more modest in its ambitions: ‘Exhortations in Guardian Style have had little effect on the number of mistakes; the level of mistakes has been little affected by our exhortations; we hope to effect a change in this’. Searching the newspaper’s website suggests that the Guide has not been as effective as it might have hoped.
There are numerous websites offering guidance on the correct use of these words, although the advice on offer can be rather confusing, or downright contradictory. That such blogs are not always helpful is apparent from one example where a reader has commented: ‘This article was very affective’. This gives rise to a lengthy discussion about whether this should be affective or effective, with no resolution. Having failed to find clear guidance after turning to such sites, James Napoli of the Huffington Post opted to hedge his bets: ‘To put it another way, what you said was very effecting. Although it may also have been affecting. Either way, it was certainly as impactful as they come’.