(google'd): "A flash mob is a group of people who assemble suddenly in a public place, do something unusual or notable, and then disperse. They are usually organized with the help of the Internet or other digital communications networks."

My favs:
http://www.youtube.com/watch?v=nhV3PrZKJOg
http://www.youtube.com/watch?v=bj0Ma2CsHME&mode=related&search=

Some ad with one:
http://www.youtube.com/watch?v=RXFceSFbSng&NR=1