The most popular of three standards for 100Mbps ethernet. (the other two being 100BaseT41 and 100BaseFX)
The 100BaseT family of protocols, transmit data over twisted pair cable. 100BaseTX transmits over cat5 or greater UTP cable, with RJ45 plugs, wired in the same way as 10BaseT. (see rj-45 for how to wire one). It uses two of the four pairs, each clocked at 100 MHz, one to transmit (1,2) and one to receive(3,6). The other pairs ( (5,4) and (7,8)) do nothing.2
100BaseTX provides a point-to-point connection, connecting only two devices to each other. To build a network of more than two computers, a hub or switch must be used. 100BaseTX cards can generally also use 10BaseT, but repeating hubs must run every port at either 100BaseTX or 10BaseT. Switches can 'mix and match' 100BaseTX and 10BaseT for each port.
100BaseTX can communicate in half or full-duplex modes. One pair is used to transmit/receive data, and the other is used to detect collisions. When there is no possibility of collision (typically when connected directly to another NIC using a crossover cable, or connected to a switch), both pairs can be used at the same time for full-duplex operation.
1 - 100BaseT4 is similar to 100BaseTx, but uses all four pairs, clocked at 33MHz. It is capable of only half-duplex operation, data being sent over the transmit pair, and the two other pairs. Collisions are detected on the receive pair. 100BaseT4 is very uncommon nowadays.
2 - There exist devices that multiplex another 100baseTX signal onto the unused pairs, allowing two computers to be served by one wire, which can save money in long cable runs with high-quality cable, or render both links useless with low-quality cable.