Someone who is known (either world-widely or nationally) through the internet, though mainly from Youtube. Many get hated especially if they breakthrough and be part of Hollywood business (Justin Bieber, Rebecca Black), but a lot are admired.
Most internet celebrities are comedians, singers, bloggers and, nowadays, pranksters. The singers part is where some haters grow, but the comedians get a lot of praise.
They usually start out good and funny, but after a few videos, they start to do anything they want and lose ideas/creativity. But people still like them. I admit, I'm one.
Janoskians, Chris Crocker (one of the first, most influential), Shane Dawson, Smosh, Nigahiga, MrTeddy, Kevjumba, Ray William Johnson, Perez Hilton, Jenna Marbles, Community Channel (Natalie Tran)...are examples of an internet celebrity