A local area network first described by Metcalfe & Boggs of Xerox PARC in 1976. Also known as IEEE 802.3
Data is broken into frames and each one is transmitted using the CSMA/CD algorithm until it arrives at the destination without colliding with any other packet. A station is either transmitting or receiving or idle at any instant. Newer equipment supports full-duplex, where a station can transmit acknowledgments without halting a receive. Full duplex requires a switch with support, rather than a hub. The bandwidth is from 10 Mbit/s (ethernet) to 100 Mbit/s (fast ethernet) to 1000 Mbit/s (gigabit ethernet.) Some time ago, a 10 Gbit/s ethernet standard was agreed upon, and it is now (2002) possible to buy commercial equipment for this. Somewhere.
Ethernet cables are classified as "XbaseY", e.g. 10Base5, where X is the data rate in Mbps, "base" means "baseband" (as opposed to radio frequency) and Y was originally the maximum cable run from end to end (500m for 10Base5, nearly 200m for 10Base2, 100m for 10BaseT), but the introduction of Fibre and 1000BaseT, the T more seems to refer to Twisted Pair these days. The original cable was 10Base5 ("full spec"), others are 10Base2 and 10BaseT which is now (2002) dissappearing. 100BaseT is the desktop standard, and 1000BaseT is quite reasonable for servers.
Ethernet is at LAYER TWO - DATA LINK LAYER of the OSI model.