Buzz or word of mouth marketing defined

A very successful word-of-mouth promotion creates buzz. Buzz generates a highly intense and interactive form of word-of-mouth referral that occurs both online and offline. Successful word-of-mouth initiatives do not follow a strictly linear process with information flowing from one individual to another rather successful models leverage subgroup connectivity and relationships by pursuing a Reed's Law hub approach to message distribution. A marketer has successfully created buzz when the interactions are so intense that the information moves in a matrix pattern rather than a linear one. The result is everyone is talking about or purchase the product or service.

A very good example is Google itself
Gmail - Google did no marketing, they spent no money. They created scarcity by giving out Gmail accounts only to a handful of "power users." Other users who aspired to be like these power users "lusted" for a Gmail account and this manifested itself in their bidding for Gmail invites on eBay. Demand was created by limited supply; the cachet of having a Gmail account caused the word of mouth, rather than any marketing activities by Google.