The tailhook is a strong metal bar, with its free end flattened out, thickened somewhat, and fashioned into a claw-like hook.
The pilot is capable of raising this bar to its inflight position or lowering it ("Hook Down") for landing.
Aircraft land on the flight deck of an aircraft carrier on a short "landing area", towards the aft end of the ship.
A number (usually four) of very greasy and very strong thick flexible cables called arrestor cables, "arresting wires" or "cross deck pendandts" are stretched across the landing area.
When an aircraft lands properly, the tailhook should catch one of the cables and the cable will transfer the energy of the aircraft, through a "purchase cable," to the belowdecks arresting gear engines, stopping the aircraft.
Both the hook and cables have to be very strong: aircraft land on carriers at full throttle, so as to have enough power to go around if the hook misses all the cables.
Source Wikipedia - Edited by me