1000BASE-T is a standard for Gigabit Ethernet. While 100BASE-T Ethernet only uses 2 pairs of the cat5 cable, 1000BASE-T uses all 4 wire pairs and a different modulation technique.

100BASE-T uses one twisted wire pair for transmit and one for receive exclusively in a three level MLT signalling scheme.

1000BASE-T uses all 4 pairs for send and receive in a 5-level pulse ampitude modulation (PAM-5) technique.