Ethernet (this name comes from the physical concept of ether) is a frame-based computer networking technology for local area networks (LANs). It defines wiring and signaling for the physical layer, and frame formats and protocols for the media access control (MAC)/data link layer of the OSI model. Ethernet is mostly standardized as IEEE's 802.3. It has become the most widespread LAN technology in use during the 1990s to the present, and has largely replaced all other LAN standards such as token ring, FDDI, and ARCNET.
More Detailed Information can be found at Wikipedia.
Ethernet in ThinkPads
Ethernet is available in various ThinkPads starting with the first A, T and X models.
Have a look at the detailed list of Ethernet Controllers built into ThinkPads.