Sybil Attack is a type of attack seen in peer-to-peer networks in which a node in the network operates multiple identities actively at the same time and undermines the authority/power in reputation systems.
The main aim of this attack is to gain the majority of influence in the network to carry out illegal(with respect to rules and laws set in the network) actions in the system. A single entity(a computer) has the capability to create and operate multiple identities(user accounts, IP address based accounts). To outside observers, these multiple fake identities appear to be real unique identities.