Autobrake refers to the system that automatically applies brake pressure to obtain a target rate of deceleration. It can be switched in or out as required by SOP or at the pilot's discretion. Air/ground relays/logic and thrust lever position determines when the system will activate, if switched on. It will not activate until the wheels 'spin up', indicating to the computer via sensors that they are on terra firma and the tires have traction.
Anti-skid refers to a system that enhances braking action, particularly on wet or slippery tarmac, and when high brake pressures are applied for rapid deceleration. Anti-skid rapidly cycles brake pressure to the wheel or wheels, allowing the tires to maintain traction near the limits of adhesion. A rapid decrease in wheel speed, as measured by sensors at the wheel, activates the system. On some newer aircraft, a wheel's speed is not only compared temporally against itself, but also to other wheels on the aircraft.