A Viral Marketing campaign is a Form of Link Baiting. I would define a Viral campaign as sneaky marketing. What you get is indirect marketing of the brand through a device or vehicle that people find entertaining and interesting. Usually a subtlety branded video or website, if done effectively, is a good way to spread your message under the radar, so to speak. These campaigns rely on pre-existing Social networks, like twitter, Facebook and even word of mouth to spread. The classic example is the the subservient chicken website that was created for Burger King. Remember for Search engine Marketing purposes, the Goal is link baiting and providing an easy way for users to link to your Viral marketing campaign.