On the Internet, viral marketing is any marketing technique that induces Web sites or users to pass on a marketing message to other sites or users, creating a potentially exponential growth in the message's visibility and effect. One example of successful viral marketing is Hotmail, a company, now owned by Microsoft, that promotes its service and its own advertisers' messages in every user's e-mail notes. (From: Whatis.com)
As I understand, viral marketing is something like a principle of Word of mouth, through making good use of the word of mouth to spread the marketing message or information. And the message will disseminate like a virus, rapidly spread far and wide. For viral marketing, the marketing message may contain "benefit to user" or "gimmick" element, to attract user's interest and let them more voluntarily pass on message to other user.
