A helicopter is a type of rotorcraft in which lift and thrust are supplied by rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward, and laterally. These attributes allow helicopters to be used in congested or isolated areas where fixed-wing aircraft would usually not be able to take off or land.
A helicopter is a kind of aircraft where the airfoils that generate lift are attached to one or more vertical or near-vertical spinning shafts, forming the helicopter's main rotor assembly or assemblies. This allows a helicopter to remain airborne even at low or zero forward speed, whereas a fixed-wing aircraft has to maintain a high forward speed in order for its wings to produce sufficient lift to keep it in the air.
As a result, helicopters can take off and land vertically, and, instead of needing a long runway, they only require a small, circular helipad, not much larger than the helicopter itself. Helicopters are, therefore, invaluable for operations in dense urban or forested areas (where it would generally be impossible to find sufficient room for fixed-wing aircraft to take off or land), to or from small, remote sites such as offshore oil platforms (ditto), and for military close air support (where a helicopter can hover over the battlezone and provide continuous fire in support of ground units, whereas a fixed-wing aircraft has to fly over in multiple short passes that can leave gaps of no air support in between them). Helicopters can also hover in midair above the ground, which allows them to be used to pick up and/or deliver supplies, equipment, and/or personnel without needing to land, if need be.
Helicopters with only a single main rotor need some way of keeping the torque from the spinning rotor from spinning the helicopter itself around; the most common way of doing this is with a smaller tail-rotor that blows air sideways, but other methods also exist.