Hardware or software designed to block/filter certain types of network traffic.
One software firewall is ZoneAlarm, which I use. Software firewalls are typically used/needed by consumers with full-time net connections (e.g. Cable Modem, DSL). Brick House is the MacOs X equivalent, but I find it much less convenient to adjust.
Hardware firewalls (e.g. Cisco) guard an entire network of machines with a piece of hardware. You can block specific ports, and do all sorts of other weird things.