DC++ is a peer-to-peer file sharing client that uses the Direct Connect protocol.
Direct Connect (DC) works by using individual servers, called hubs, to hold information about the files its users want to share. Click View > Public Hubs to display the various hubs you can access in DC++; a quick double-click lets you connect to them, maybe search for files, chat with other users and more.
Much of this works very well. There's excellent firewall and router support, for instance, and we found selecting the UPnP option allowed DC++ to configure itself in just a few seconds. It's also straightforward to keep track of your favourite hubs, automatically connecting to them when the program is relaunched. And your downloads can be paused and resumed, too.
The DC world is very different to other peer-to-peer setups, though. Individual hubs have their own rules and etiquette, for instance: some will let you connect and search right away, others may not allow searching immediately, or you might not be allowed to connect at all unless you're sharing a certain amount of data. These issues and plenty more are covered in the very detailed documentation, but expect to spend quite some time reading this before you'll be able to find your way around.