It's not that people don't know what these words mean; it's that they have trouble remembering which one does what. The simplest distinction here is that affect is almost always a verb, and effect is usually a noun.
These words are frequently confused in part because their meanings are related. For more information, please see our Ask the Editor video.
"Lithium, after all, can be toxic, and though the levels in the Oita study are too low to have an immediate effect, the element can affect kidney function and cause long-term health problems." Clay Risen, The New York Times Magazine, December 13, 2009