A meme is a type of online chain letter where bloggers answer questions or participate in a quiz designed to give a quick overview of the author’s personality.Once the author completes the meme, it is customary to tag other bloggers to participate.

An internet meme (also known as internet phenomenon) is the sudden popularization of something due to a number of bloggers posting about it.

Meme resources:
WikiPedia: Meme
WikiPedia: Internet Phenomena

A few memes at a few sites:
Four Things
You Are What You Eat
Successful Habits