The Sonic Screwdriver is a versatile tool and defensive device first used by the Doctor in his second incarnation for opening up hatches, panels, and controls.
Later uses included cutting through wall sections and even as a conventional screwdriver. During the Third Doctor's exile on Earth he had a larger and more elaborately detailed version, which had a decorated silver and black handle and black and yellow stripes. This version had even more functions including detecting security traps, remote detonation of mines, opening doors and restraints, creating ignition sparks and again removing screws and fixings.
Later still the Fourth Doctor remade yet another similar style of Sonic Screwdriver but with many more features and functions including micro soldering of circuitry, cutting wires and bulkheads, and even detonating Dalek explosives.