Direct Connect is yet another variation on the IRC + DCC theme. It is adware, although there is a free-software variant called DC++ that has no ads.
What is very peculiar about Direct Connect is its somewhat "elite" culture. There are hubs that do not let you in if you don't have at least 40 Gigabytes shared and 10 download slots open. Usually those users also have massive bandwidth. Direct Connect is where a lot of trading of DIVX movies, warez and ISO images of whatever you may imagine happens. There is a certain amount of pr0n, but it is not the main content really.
Notice that if you are behind a firewall or/and a NATting router you can connect only in passive mode, which means that multi hub searches do not work.
See also: Napster (requiescat in pacem), Gnutella, Kazaa, Morpheus, eDonkey, AudioGalaxy, SwapNut.