This document defines and discusses viral marketing. It begins by explaining that viral marketing is a strategy to spread marketing messages through social media where the messages multiply rapidly and are sent to thousands, similar to how viruses spread. It then lists some common viral marketing tactics like video clips, games and images. The document outlines advantages like large audience reach and low costs, and disadvantages such as potential annoyance or negative reputation. Finally, it provides examples of successful viral marketing campaigns from companies like Hotmail, Blendtec and Mentos.