A meme is an idea, thought or behaviour that is passed from one person to another through imitation, although the ideas or behaviours can mutate or change as they spread. The spreading can be compared to the spread of a virus. In web 2.0, thoughts, questions, music and information is posted on the internet and responded to, changed and expanded on by others in blogs, wikis and other methods. Think "broken telephone" for the internet age.
Wikipedia lists some examples of memes: music, literature and video; slogans, expressions, jokes and quotations; traditions, theories, medical advice, urban rumours and hoaxes, viral marketing.
Examples of Memes:
http://ytmnd.com/ - probably not suitable for the classroom, but one of the top examples
Dipity.com - create timelines that can be embedded in a wiki, website or blog. You can also geo-tag events on a Google Map so you can show events chronologically and geographically, helpful for history. You can also attach video, news stories and images from various sites.