(Part1) Definiton of Viral marketing
Do you know the definition of viral marketing? If you are unclear, now is the time to pay attention, because what you don't know could be negatively affecting your business.
Cyber space is abuzz with internet viral marketing and how to boost one’s business through a viral marketing campaign. The most common and basic definition of viral marketing offered by the search engines is that it is spreading a message (much like a virus; hence the word “viral”) about one’s products or services as a way to promote them.

• Word of Mouth and Internet Viral Marketing

While this “spreading the word” concept about viral marketing products and services is an important component of viral marketing, it does not fully explain this marketing phenomenon. An important link is missing there, and that is the consumers and how their personal opinions about one’s promotional messages or goods can make or break the marketer’s efforts to go viral.
The most important question that viral marketers must ask themselves when crafting a viral marketing campaign, therefore, is what’s in it for the consumers. Why should people even bother to spread their message to others?

• The Definition of Viral Marketing Uncovered

Catching the kind of “virus” that is worth infecting others with is crucial because consumers can always decide to just keep it to themselves much like someone with a flu would shun human contact for days.
Another important element of viral marketing—it’s the most important, actually; it’s why marketers are going ga-ga over viral marketing!—is that the message must bring about the ultimate desired effect: increase in sales through brand awareness. And one of the most popular ways of spreading this message is through starting an ezine and viral email marketing.
http://www.manifest-passive-income.com/definitionofviralmarketing.html

• Definition of Viral Marketing ( from Wikipedia)

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.
The goal of marketers interested in creating successful viral marketing programs is to identify individuals with high Social Networking Potential (SNP) and create viral messages that appeal to this segment of the population and have a high probability of being taken by another competitor.

The term "viral marketing" has also been used pejoratively to refer to stealth marketing campaigns—the unscrupulous use of astroturfing on-line combined with undermarket advertising in shopping centers to create the impression of spontaneous word of mouth enthusiasm.

http://en.wikipedia.org/wiki/Viral_marketing