Here's a great little video on what Twitter is, in laymans speak. Really good overview. When people start using Twitter, the big problem is understaning what it is. Because the thing that made Twitter viral is also its biggest obstacle to user adoption. People just don't get it straight away.
In fact, check out CommonCraft's YouTube channel here. They do plain engligh' explanations of blogs, wikis, bookmarking and RSS, to name a few.
The Twitter video's pasted below too: