Its a marketing phenomenon that facilitates and encourages people to pass along a marketing message. Mean other people will help you promote your site.
Hi, it's a way to self-promote your website. In other words, your website starts promoting itself. This can be done in various ways, i.e. creating a plug-in that people just love to post on their blogs/websites that has a built-in link to your website, creating a program that's really useful to people that has your link build in, etc.
Viral marketing is the way of marketing where you can get lots of visititors/signups in a short period of time I wrote few articles about it heare are the links http://hubpages.com/hub/5-ideas-for-viral-marketing http://hubpages.com/hub/Why-Viral-Marketing-is-so-Powerful