The difference between a viral marketing campaign and a conventional marketing campaign is mainly in how the message is spread. A conventional marketing campaign tries to find out where the target audience is, and then makes sure their ads appear in those places. Examples of this basic strategy are pretty straightforward: a make-up ad in a fashion magazine, a local lawn mowing service putting up a flier at the neighborhood supermarket, a car company buying a banner ad on a car enthusiast website, and so on.
Viral marketing campaigns depend on other people to distribute the advertising message through social media networks. In theory, an individual shares an interesting piece of marketing content with all of his or her network; that person's friends then pass the message on to their networks, creating a snowball effect. (Learn more about social sharing in Understanding Social Media: What You Need To Know.)
Viral marketers often claim that a viral campaign is largely for branding and needs to have substantially different content than conventional marketing. However, with the range of approaches that conventional marketing has at its disposal, this distinction is becoming less and less clear. As many TV advertisements have shown by going on to be YouTube hits, conventional marketing can be just as shareable as marketing that is specifically designed to be viral. In addition, it is very difficult to predict what will become viral, which makes it risky for an advertiser to make becoming viral its only goal in a given campaign.
The main difference between viral marketing and conventional marketing is that viral marketing depends on social sharing, whereas conventional marketing depends on delivering the message to the target audience.