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):
When “affect” is accented on the final syllable (a-FECT), it is usually a verb meaning “have an influence on”:
“The million-dollar donation from the industrialist did not affect my vote against the Clean Air Act.”
“to make a display of or deliberately cultivate.”
Occasionally a pretentious person is said to affect an artificial air of sophistication.
Speaking with a borrowed French accent or ostentatiously wearing a large diamond ear stud might be an affectation.
“emotion.” (when the word is accented on the first syllable (AFF-ect)).
In this case the word is used mostly by psychiatrists and social scientists—people who normally know how to spell it.
When you affect a situation, you have an effect on it.
Less common is a verb meaning “to create” (to cause to come into being):
“I’m trying to effect a change in the way we purchase widgets.”
The verb effect goes beyond mere influence; it refers to actual achievement of a final result.
Here’s a quick informal technique for you: If it is not easy for you to remember that that the word “affect” is most commonly used as a verb while “effect” is usually used as a noun, then label this confusion as “aven.” It sounds like amen. The “av” in aven should make you recall affect as verb and the “en” is effect as noun.