viral marketing is in a sense "word of mouth" advertising. Your message is circulated from friend to friend and takes on a snowball effect, driving free traffic back to your site with very little effort on your part.
It can be an excellent way to drive traffic since people are more likely to act on a recommendation from a close trusted friend as opposed to receiving an email message from someone they don't know and haven't established a relationship with.
Check out my hubs for more info. I have completed a 3 part series, outlining 21 very effective ways to use viral marketing to increase traffic to your site :)