The buzzwords viral marketing and viral advertising refer to marketing techniques that use pre-existing social networks to produce increases in brand awareness or to achieve other marketing objectives (such as product sales) through self-replicating viral processes, analogous to the spread of pathological and computer viruses. It can be word-of-mouth delivered or enhanced by the network effects of the Internet.[1] Viral promotions may take the form of video clips, interactive Flash games, advergames, ebooks, brandable software, images, or even text messages.
http://www.youtube.com/watch?v=Oe3St1GgoHQ