What is the difference between twin and earth cable and SWA cable?

Twin and earth cable (the flat grey cable you see in walls and ceilings) is the standard cable used for most household wiring — lights, sockets, cookers. It has two conductors plus an earth wire inside a PVC sheath. It's designed for indoor use only, protected by the building structure. SWA cable (Steel Wire Armoured) looks completely different — it has steel wires wrapped around it for mechanical protection. It's used where a cable might be damaged: in the ground to power an outbuilding, on an external wall, or anywhere where it might be knocked or dug up. SWA is also used in garages, workshops, and anywhere the cable is exposed. If you're running power to a garden shed or garage, SWA is almost always the right choice — twin and earth cannot be buried directly in the ground without conduit.