"Ethernet" is the term that is casually applied to a number of very different data link implementations.  You will hear people refer to "Ethernet" and they might be referring to the original DEC, Intel and Xerox implementation of Version 1 or Version 2 Ethernet.  This, in a sense, is the "true" definition of "Ethernet".  When the IEEE built the 802.3 standards in 1984 the term "Ethernet" was broadly applied to them as well.  Today we talk about "Fast Ethernet" and, although this technology bears many similarities to its predecessors, the engineering technology has changed dramatically.

Whatever you call it, this is a Data Link technology - responsible for delivering a frame of bits from one network interface to another - perhaps through a repeater, switch or bridge.

SubCategories:

Ethernet Frame Formats
Analysis of Ethernet Frame Formats
Fast Ethernet
Analysis of Fast Ethernet