Viral Marketing is a marketing phenomenon that facilitates and encourages people to pass along a marketing message. It's somehow called "word of mouth marketing". Below are the six basic elements you hope to include in your strategy. A viral marketing strategy need not contain ALL these elements, but the more elements it embraces, the more powerful the results are likely to be. An effective viral marketing strategy:
1. Gives away products or services
2. Provides for effortless transfer to others
3. Scales easily from small to very large
4. Exploits common motivations and behaviors
5. Utilizes existing communication networks
6. Takes advantage of others' resources
Source:
http://www.wilsonweb.com/wmt5/viral-principles.htm