reputation system

• Sybil attack

The Sybil attack in computer security is an attack wherein a reputation system is subverted by forging identities in peer-to-peer networks
It is named after the subject of the book Sybil, a case study of a woman diagnosed with dissociative identity disorder.

reputation system
A reputation system computes and publishes reputation scores for a set of objects (e.g. service providers, services, goods or entities) within a community or domain based on a collection of opinions that other entities hold about the objects
http://en.wikipedia.org/wiki/Sybil_attack