Viral marketing can refer to either an offline or online process. Online viral marketing campaigns are becoming increasingly popular. They involve quite a lot of research into which segments of social networks your product or brand is going to appeal to. These need to be groups with a high social networking potential (SNP). Once you have identified your audience, you need to provide them with something that they'll want to share... it's even harder than it sounds.

What makes viral marketing so difficult?

Basically, it's that you can't force people to listen to what you have to say. You have to make what you're saying of interest to your target audience enough that they'll want to pass it on to their friends. Basically, you need to practice being popular.

An example of a successful marketing campaign:

Ronaldinho: Touch of Gold (Nike)

Football legend, Ronaldinho gets given a new pair of boots then spends the next two minutes showing off his amazing football skills. Over 23 million people watched this ad online.