Which encoding is used in Ethernet LAN?

Ethernet indicates 10 Mb by definition. It used Manchester encoding. This encoding manages timing within each bit.

FastEthernet is 100 Mb and uses a combination of 4b/5b and MLT-3 encoding, over 2 pairs.

Gigabit Ethernet uses 8b4qt (?) and PAM-5

I'm not sure I got all this right, but here are the details as I understand them:

10 BASE T is Manchester encoding - Basically, this means that each symbol is either a transition in the middle of the bit time from a 0 to +v (a 1) or a transition in the middle of the bit time from +v to 0 (a 0).

100 BASE T uses a combination of 4b/5b encoding to prevent a string of 1s or 0s from causing a loss of timing and MLT-3 signalling to allow the same information to travel at a slower frequency.

1000 BASE T uses a combination of 8b4qt (?) (this is 8 bits, separated onto 4 quernary signals that carry 2 bits each), and PAM5 signaling (a 5 voltage-level signal that effectively causes a transition at each signal and 4 possible states, signalling 2 bits).