Short for Realtime Blackhole List, a list of IP addresses whose owners refuse to stop the proliferation of spam. The RBL usually lists server IP addresses from ISPs whose customers are responsible for the spam and from ISPs whose servers are hijacked for spam relay.
As subscribers to the RBL, ISPs and companies will know from which IP addresses to block traffic. Most traffic blocking occurs during the SMTP connection phase. The receiving end will check the RBL for the connecting IP address. If the IP address matches one on the list, then the connection gets dropped before accepting any traffic from the spammer. Some ISPs, though, will choose to blackhole (or ignore) IP packets at their routers. The goal here is to block all IP traffic.
It is important to note that all e-mail and packet blocking is done by the recipient, not the RBL administrator, which is only responsible for bouncing spam that is directed at its servers.
The RBL was created by Mail Abuse Prevention System (MAPS) LLC., but there are other entities that keep RBLs aside from MAPS.