Nickelodeon is the channel that financed and aired Invader Zim. They later cancelled the show and put it on Nicktoons, as they did with most syndicated television shows formerly televised on Nick. Nickelodeon is popular for other shows (other Nicktoons) including SpongeBob SquarePants and The Fairly Oddparents.
According to many commentaries and interviews, Jhonen Vasquez and Nickelodeon clashed on many ideas, such as Keef's and Dib's deaths, the inclusion of Bloody GIR, and some minor stereotyping (such as the ending of Walk of Doom, in which Vasquez wished for Zim to end up in Mexico, but Nickolodeon feared it would be offensive. Vasquez stated in the commentary that he does, however, like the new version more).
In response to these restrictions, the creators of Invader Zim slipped in many ways to poke fun at Nickelodeon, or to simply go against their wishes. This includes Nick, the incredibly happy boy (who is a not-so-subtle reference to Nickelodeon's constant demands that Jhonen make the show a bit "happier" and more appealing to certain television viewers, and is coincidentally voiced by the creator himself), and the infamous Bloody GIR, which was slipped in at random, one-framed intervals in several episodes.