Definition:
A LAN or Local Area Network is a computer network (or data communications network) which is confined to a room, a building, or a group of adjacent buildings. A similar network on a larger scale is sometimes referred to as a WAN (Wide Area Network), or in some cases more specifically, a MAN (Metropolitan Area Network) if it is confined to a single metropolitan area.
The term LAN is most often used to refer to networks created out of a certain class of networking equipment which is tailored to communication over a short distance. This is in contrast to networks which happen to span short distances, yet are constructed using "WAN" equipment (i.e., equipment capable of transmitting long distances). LAN-style networking equipment typically transmits data at a higher rate than WAN-style equipment: the equipment's design takes advantage of the short distance to supply a high transmission-rate at a relatively low cost.
If you are familiar with network access using a modem and ordinary telephone line, note that both LAN and WAN equipment typically offers faster data transfer than even the fastest ordinary modem/phone-line access, LAN transfers being on the order of a million times faster. This means graphics that are loaded through the network can be displayed significantly faster, and that there are things that it is practical to do on a LAN that you would never do with a modem: for example, you might set up your computer to load your word processing application through the LAN rather than from hard disk; the time you have to wait while it loads would be similar (a few seconds) in either case. In contrast, loading such an application through a modem would require minutes or hours.
A typical use of a LAN is to tie together personal computers in an office in such a way that they can all use a single printer and a file server (briefly, a file server is a computer set up so that other computers can access its hard disk as if it were their own). LANs are also used to transmit e-mail between personal computers in an office, or to attach all the personal computers in the office to a WAN or to the Internet.
There is some variation in the way the term LAN is used:
For example, someone once asked me if an Ethernet can do the same thing as a LAN, like provide you with disk space. In the more specific sense of the word LAN, an Ethernet is a type of LAN. In the broader sense, and in the sense that this person was probably using, an Ethernet is a portion of the equipment used to create the entire LAN.
This website deals with this latter, more specific, sense of the term. You can find out more about LANs in the broader sense of the term from pages by vendors of file servers (e.g. Novell, Microsoft, Apple, etc.).
Definition:
Some other types of LANs (or in the jargon: other LAN technologies) are Token Ring, FDDI, Fast Ethernet. Ethernet is the most common type in use today. Equipment of two different technologies cannot be plugged together without a special device (bridge or router) between that can translate from one technology to the other. For example, if your file server has an Ethernet adaptor and your PC has a Token Ring adaptor, they cannot be interconnected by a piece of wire, but if each were attached to the same bridge (specifically, one that translates between Ethernet and Token Ring; it might cost a few thousand dollars), then they could intercommunicate.
This website has a number of pages for further information including a page of links to various LAN tutorials around the net. You may find browsing LAN Glossary to be of some use, though it is certain to raise as many questions as provide answers.
--John Wobus, 3/26/97