I've noticed that some people use effect and affect interchangeably. What are the differences between these two and when are the proper situations to use each of them?
The noun is usually "effect" -- unless in more formal or literary contexts in which case "affect" as a noun can mean feeling or emotion.
The verb is generally "affect", although "effect" is possible if the meaning is "put into place" or "carry out".
Here are some example sentences:
"His plans had no effect on me."
"His disconsolate eyes brought on a sad affect."
"His plans affected me."
"He effected a plan to change the world."
The "common errors" site mentions 3 different meanings for affect (verb):
The Merriam-Webster details:
The rule that mostly works it this: affect = verb, think "a" for action, wheras effect = noun, the result of the action
to remember: "a" comes before "e" in the alphabet, and you must affect something to cause an effect