Transceiver is an electronic device for connecting a computer to a baseband transmission network so that the computer can transmit and receive signals on the network. In the 1980s, transceivers were often separate devices attached to thicknet cabling using vampire taps, but today most network interface cards (NICs) have onboard transceivers.
Some Fast Ethernet NICs have a media independent interface (MII) to which an external transceiver can be connected to provide different kinds of 100-Mbps networking. This allows you greater flexibility in your networking options. For example, 100BaseTX transceivers have an RJ-45 port for connecting unshielded twisted-pair (UTP) cabling, and 100BaseFX transceivers have an SC-type port for connecting fiber-optic cabling.