What exactly is Usenet?

I've heard about it several times in the past year, primarily that it's going to outclass Bittorrent eventually.  But it seems I'd need an entirely new ISP or something to use it?  What exactly is it, and how does one access it?  If it makes any difference, I'm using Optimum Online.

Usenet is what we had before the web was invented. Possibly before the internet for that matter.