Ethernet

Definition
Ethernet is a family of frame-based computer networking technologies for local area networks (LANs). The name comes from the physical concept of the ether. It defines a number of wiring and signalling standards for the Physical Layer of the OSI networking model, through means of network access at the Media Access Control (MAC) /Data Link Layer, and a common addressing format.

Usage In Lighting
In lighting it is used to control/monitor some dimmers, as well as communicating with RFR's, WYSIWYG software and lots more.

Parts taken from:
http://en.wikipedia.org/wiki/Ethernet