A drone is an unmanned remotely controlled flying vehicle that also can be used for specific work in addition to just being able to fly around. Drones are used for photographic and video recording from above. Some are used to perform scientific or technical measurements. Some carry weapons and are used as combat tools. Still other are used to carry cargo and post between places. The latest development are drones that carry tools for installation and repair.
The basic technology in a drone consist of some kind of power plant, some propulsion mechanism, some kind of steering mechanism, some kind of sensors to recognize place and path and a sender-receiver unit to transmit and receive signals for steering and recording. Drones are found in all shapes you can find in greater airplanes, and also in all kind of exotic shapes, like oblong airships, disks, triangles, donuts, stars or can resemble big insects or birds.
Small or light drones are typically driven by propellers connected to electric motors powered by batteries. Steadily more one produces very light drones, powered by solar panels, that can fly in the air for several weeks in principle. Some drones are held aloft by gas lighter that air, like a blimp, which also makes the drone able to hold itself in the air for a long time. Greater or heavier drones are mostly powered by some kind of combustion engine, like piston motors, turboprop motors or turbojet motors drone photography.
It is also possible to construct drones powered by nuclear energy that can stay aloft for days or weeks, and especially if those drones are partially made as gas-filled blimps. Such nuclear power sources need not be what is considered atomic reactors. The energy source can be special isotopes that radiate intensely and thereby produce enough heat to power the drone. Some of these isotopes radiate mostly alpha-rays or beta-rays that can be shielded off without heavy armors.
Since most uses of drones are held secret or simply not announced publicly, it is difficult to give a full account of the extent of their use and who are using them, but a fairly clear picture emerges based on official sources, journalistic and scientific articles, commercials from drone producers, and stories of people coming in touch with drones in use.
Most drones are probably used by civil authorities and property owners for domestic surveillance of various kind, like border control, road traffic surveillance, wildfire detection, air photography, geographic mapping, pipeline control, electric grid control and surveillance of crowd behavior. Drones are steadily more used to transmit views of sport championships. Police forces use drones for detection and investigation of crimes, a practice that also seems to involve direct spying by drones on private citizens at their homes.