In everyday speech, affect is a verb. It means to influence something, such as in the headline from the Albuquerque News,
Downed Power Line Affects PNM Customers
The downed power line had an impact on some power customers: they were without electricity overnight.
Effect is most commonly used as a noun meaning the result or impact of something, an outcome. If there's "a/an/the" in front of it, it's an effect. The second sentence is from a story about the outcome of long-term sleeping trouble,
The Effect of Persistent Sleepiness
One of my biggest pet peeves with my employees. Thank you!