Viral marketing induces Web users to pass on a marketing message. Viral marketing is a specific promotional strategy which relies on individuals to spread a message, call for a response, or induce an action or reaction.

Viral marketing not actually "marketing" - it is a descriptive term to describe promotional strategies that rapidly spread a message with voluntary and discretionary involvement of any group, its members and group opinion leaders.

Generating Viral Marketing Promotion: How to Start a Viral Marketing Campaign