You can either use Torrents or some peer-share network (I know more about the second). There are also binary files, but I’ve never used that, so I can’t comment.

The content available is supplied by other users – there is no one website that finds and supplies the video. Here’s how it works. User-A wants video. He connects to a network and finds that the video he’s looking for is available on special folders of the HD’s of other users. He can queue up behind other people who want content from that source. The source may not want to waste bandwidth downloading to User-A unless User-A offers the same courtesy to others. Therefore, User-A might be expected to already have some content of his own (on pain of getting permanently kicked from Source’s que if he doesn’t). This is actually pretty easy since the price of content has pretty much dropped through the cellar floor in the past 5 or so years. There’s no real excuse for leeching in this day and age.

Because the users supply all the content, QA is pretty much a moot question. You’ll get DVD rips (good) or you may get the dreaded shot-in-the-theater video (horrible quality, poor perspective and yeverybody who goes to the bathroom will have his shadow immortalized). The best way to avoid that trap is to wait until the DVD release. If you see a movie available on the network that’s still in theaters, it’s likely a horrible bootleg and should be avoided.