100baseTX runs over two pairs of wires in category-5 cable. Like 10BASE-T, the proper pairs are the orange and green pairs (canonical second and third pairs) in the TIA-568B wiring standard.
In TIA-568B, wires are in the order 1, 2, 3, 6, 5, 4, 7, 8 on the modular jack on each end. The colour-order would be orange/white, orange, green/white, blue, blue/white, green, brown/white, brown.
Each segment can have a maxium distance of 100 metres. Capable of 100Mbps throughput. See IEEE 802.3 for more details.
See also:
Based on material from FOLDOC, used with permission.
wikipedia.org dumped 2003-03-17 with terodump