A person (or persons) that one may typically find on the internet. Their opinions and understandings of any subject they choose to discuss are superior to that of you or any expert in the field. They frequent message boards, the blogosphere, and can be found anywhere on the internet. Not every person that uses the internet is an "internet person," and it's very rare that an internet person would acknowledge his or her self as being one.
Internet people can easily be likened to the Comic Book Guy from The Simpsons.
They also frequently like unpopular things and hate popular things, though if unpopular things become popular they can switch their opinions easily.
Normal person: "I loved Ocarina of Time, it's easily one of the greatest games of all time."
Internet person: "Ocarina of Time was a trite play on what was once a great series. It is simply A Link to the Past in 3D, with a simplistic and predictable story. Your opinion on the game shows your lack of sophistication."