Clip-ons are simply replacement handlebars for sportbikes. The advantage is that they are easily replaced in the event of a crash, are adjustable (forward or back) and some adjust up and down also. I have 'em on both my bikes. I'd post pics, but can't figure out how to do it.:headscratch
Like said above, it's any "handlebar" that is for a single side only and clamps to the fork itself. The biggest advantage is that if you mess one up you only need to replace that side instead of a whole handlebar assembly, like on a different bike.
Keep a close eye on Gary Mccoy next time you watch motogp. He does that every chance he gets. And thinks it's no big deal when he does it. Some people like Rossi, some like Stoner, or Hayden. But McCoy is my hero .
Wider bars give you more leverage. This is why a lot of street fighters put dirtbike bars on their bikes. Clip ons put more weight on the front end (by forcing you to lean forwards). Making steering faster because of the forwards distribution.