As you know that an Ethernet is a LAN architecture developed by Xerox Corp in association with DEC and Intel.
Ethernet uses bus or star topology and can support data transfer rates of upto 10 Mbps.
The computers that are part of Ethernet, have to install a special card called Ethernet Card.
If the Ethernet card is designed for coaxial cable then the connection will be BNC and if it is designed for twisted pair cable also contain an AUI connector. This can be used to attach coaxial, twisted pair, or fiber optics cable to an Ethernet card.
Here are the definitions of BNC and AUI connector:
When this connection is used then there is always an external transceiver attached to the workstation.
Generally, Ethernet cards are purchased separately from a computer, but these days many computers such as Macintosh include an option for a pre-installed Ethernet card.