"Viral marketing campaigns are those that self-propagate because the target audience wants to distribute them for their inherent entertainment value or practical use." - Guy Lecky-Thompson

The above is as close as anyone is likely to get to a formal definition that covers viral marketing as we know it in Web 2.0 : it is a marketing message that spreads by itself.

The viral marketing medium that most people are familiar with is email. How often does the average email user have a video sent to them that turns out to be a YouTube video, or online flash game?

Friends like to share things that they find interesting, fun, or useful. Advertisers like to take advantage of this by combining social proof ("my friend liked it, so I should") with a marketing message delivered in a unique way.

An example of this are the XXXX Lager adverts from Australia, an example of which can be seen below.

The XXXX advertising campaigns build awareness and entertain the viewer. They're short enough to be sent via email, and risqué enough to warrant doing so.

Notice, however, that they're not so risqué that the recipient would feel uncomfortable viewing them at work or with the family.

Most, if not all, successful viral marketing campaigns contain some form of interactive, or non-interacive entertainment media. Some examples of interactive entertainment media that could be used in viral marketing campaigns are:

• Games
• PowerPoint presentations
• Web Pages with Polls
• etc.

Non-interactive viral marketing campaings could include:

• Video (via YouTube or similar)
• How-To PDFs
• Comical Animated GIFs (in email signatures)
• etc.

Anything that the first recipent wants to share, and that promotes the brand of marketing message can be considered to be viral. To be successful, the message must not get lost in the delivery, and every recipient must want to forward it.