Actually, there seems to be some real debate about what exactly Viral Marketing is and is not.
The Wikipedia offers this definition:
Viral marketing and viral advertising refer to marketing techniques that seek to exploit pre-existing social networks to produce exponential increases in brand awareness, through self-replicating viral processes, analagous to the spread of a computer virus. It can often be word-of-mouth delivered and enhanced online; it can harness the network effect of the Internet and can be very useful in reaching a large number of people rapidly.
However, I have my own definition that has served me well:
Viral Marketing is simply anytime you can get someone who does not work for your organization to promote your product.
It really is that simple. The idea behind Viral Marketing is to have your patrons sell/promote your product to other "would-be" patrons without any compensation. For example, if I asked my subscribers to provide me with their friend's contact information so I could call to see if they would be interested in subscribing, that is NOT viral marketing--it is simply datamining, or direct marketing.
Now, if I could convince my subscribers with a flyer that says "Pick your friends" to sell the seats to the right and left of them and then send me the money--that IS viral marketing. We have patrons who do not work for me actaully selling my product!
SOME TYPES OF VIRAL MARKETING
The Pass Along
A message which encourages the user to send the message to others. The crudest form of this is chain letters where a message at the bottom of the e-mail prompts the reader to forward the message. More effective are short, funny clips of video which people spontaneously forward. Many of these, such began life as TV commercials and have since circulated on the web by word of mouth. The number of people reached in this way is often much greater than the number who viewed the original ad.
A reward is offered for either passing a message along or providing someone else's address. This can dramatically increase referrals. However, this is most effective when the offer requires another person to take action. Most online contests offer more chances of winning for each referral given; but when the referral must also participate in order for the first person to obtain that extra chance of winning, the chance that the referral participates is much greater.
A viral message presented as a cool or unusual page, activity, or piece of news, without obvious incitements to link or pass along. In Undercover Marketing, it is not immediately apparent that anything is being marketed. Particular effort is made to make the discovery of the item seem spontaneous and informal, to encourage natural memetic behavior. Because of the large amount of unusual and entertaining content on the internet, this can be the hardest type of viral to spot, especially as companies try to imitate the style and content of amateur websites and authentic underground movements. (Consider some of the latest examples of this trend such as the Coke/Mentos Video, The Sony Ericsson Phone Tourists)
Edgy Gossip/Buzz marketing
Ads or messages that create controversy by challenging the borders of taste or appropriateness. Discussion of the resulting controversy can be considered to generate buzz and word of mouth advertising. Prior to releasing a movie, some Hollywood movie stars get married, get divorced, or get arrested, or become involved in some controversy that directs conversational attention to them. An alleged example is the publicity campaign about the dubious love affair between Tom Cruise and Katie Holmes that came out just before each of them released a movie.
Users create and manage their own lists of contacts using a database provided by an online service provider. By inviting other members to participate in their community, users create a viral, self-propagating chain of contacts that naturally grows and encourages others to sign up as well. Examples of such services include anonymous matching services like eCrush, business contact management services like Plaxo, and other social databases like MySpace, FaceBook, Evite and Classmates.com.