Because the difference is phonetically subtle, these two words are confused by probably half of the English speaking population. In fact, I was asked about their correct usage by a colleague just this morning.
Affect is a verb. A doing word. As such, your psychiatrist might use affect in the following way:
“Tell me, then, how did the discovery that your father was a transgendered homeless schizophrenic affect you?”
Effect, on the other hand, is a noun. A thing word. It serves as the subject or object of a sentence or phrase. You might use effect when replying to your psychiatrist like this:
“I guess it was the lipstick that bothered me most, all gummed up around his unshaven whiskers. The visual effect was powerful and disturbing.”
Another difficulty with this pair of words is that their meanings are similar. While they serve two different grammatical purposes (verb vs. noun) they both refer to the same concept of impact or influence.
Perhaps a good way to remember the difference between them is by observing the word effective. Effective is an adjective, which is a word that can only be used to describe a noun. Effect is a noun and the root word of effective, which describes only nouns.
Effect and Effective are now buddies in your mind, because they live in noun-land where only nouns and words that describe nouns can live. They’re neighbors and they wave hello to one another in the morning across the street as they step out the front door in their housecoats to fetch the paper from the stoop. They carpool to work where Effect does all the heavy lifting and Effective stands around telling him what a good job he’s doing and taking all the credit for sounding pretty.
Good luck and remember, no matter what you do, you’ll do it better with proper grammar.