Automatic locking differentials:
Like the name says, automatic locking differentials require no driver action and automatically lock the axles together when torque is applied.
Some, like the Eaton MLocker, (commonly found on GM pick-up trucks and SUVs) normally operate as an open diff, and lock automatically upon loss of traction (under 20 miles per hour), then unlock when conditions allow.
Others, like the Detroit Locker, are normally locked, automatically unlocking during turns and re-locking when conditions allow.