A specification for Gigabit Ethernet over copper wire (IEEE Std. 802.3ab). The standard defines 1 Gb/s data transfer over distances of up to 100 meters using four pairs of CAT-5 balanced copper cabling and a 5-level coding scheme.
Because many companies already use CAT-5 cabling, 1000base-T can be easily implemented.
Other 1000Base-T benefits include compatibility with existing network protocols (i.e. IP, IPX, AppleTalk), existing applications, Network Operating Systems, network management platforms and applications.
Also see the Ethernet Designations chart in the Quick Reference section of Webopedia.