A local area network (LAN) is a computer network covering a small geographic area, like a home, office, or group of buildings e.g. a school. The defining characteristics of LANs, in contrast to Wide Area Networks (WANs), include their much higher data transfer rates, smaller geographic range, and lack of a need for leased telecommunication lines.
Considering QuakeWorld a LAN usually means a LAN party.
LAN party is a temporary, sometimes spontaneous, gathering of people together with their computers, which they network together primarily for the purpose of playing multiplayer computer games. These local area networks (LANs) come in various sizes, from very small (two people) to very large (more than 10,000 people). Small parties can form spontaneously, but large ones usually require a fair amount of planning and preparation on the part of the organizer. The current world record for number of computers connected in the network at a LAN party is 10,445, set at DreamHack Winter 2007, in Jönköping, Sweden.
LAN party events differ significantly from gaming centers and Internet cafes in that they are generally bring your own computer (BYOC) and are not permanent installations, often taking place in general meeting places or residences.