A catfish is someone who creates a false online identity.
Catfishing is common on social networking and online dating sites. Sometimes a catfish's sole purpose is to engage in a fantasy. Sometimes, however, the catfish's intent is to defraud a victim, seek revenge or commit identity theft.
Either way, a catfish exploits the fact that people are often willing to ignore warning signs that a friend or acquaintance may not be who they claim to be. In an online relationship, such signs include refusals to meet in person, refusals to video chat, claims of a serious disease or injury, unusually attractive profile images, personal information that doesn't add up, or requests for money.
See also: sock puppet