Definition: viral marketing and viral advertising refer to marketing techniques that use preexisting social networks to produce increases in brand awareness, 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. Viral marketing is a marketing phenomenon that facilitates and encourages people to pass along a marketing message voluntarily. Viral promotions may take the form of video clips, interactive Flash games, advergames, images, or even text messages.
(Source: Wikipedia)