What is the difference between off load tap changer and on load tap changer?

An on-load tap-changer must not break the supply to the transformer's windings while the transformer is on load. So it has a 'make-before-break' arrangement with its contacts -i.e. one contact moves to the second tap setting, before the second contact moves away from the first tap setting, while a reactor balances the load currents when the contacts are on different tap settings. An on-load tap changer is motor-driven, and can be operated automatically or manually.

An off-load tap-changer does not require a make-before-break arrangement, because it is designed to be operated when the transformer is disconnected.