You use a client to connect to "hubs", which are hosted from a central server. Each computer connected to the hub usually has to "share" a certain amount, and you can download anything that anyone else connected to the hub has "shared". Like other P2P protocols, if more than one person has something, then you'll get it faster.
It's very popular in very large LAN environments, like universities or the NAG LAN at rAge every year, as it's far superior to Windows' built-in file sharing, but it's easy to host a hub. It isn't used much over the internet, as Bittorrent is a better technology in this environment, along with regular P2P clients like Limewire. But as you can imagine, it has its appeal in South Africa's local internet loop, although torrents and Usenet are still more popular.