Nyan cat is the most awesome annoying thing on the internet. You can simply listen (and watch) for as long as you can.
Simply said, Nyan cat is simply a cat, shaped like a pop tart (or toast, depending on where you are), dancing while pooping rainbows. A song is going on, and if you hit the lower volume button, it will get louder, and if you hit it a few more times, it goes all rainbow strobe in the background. It is awesome. Do not do near epileptic people or anyone easily angered.