A drone is an aircraft with no pilot on board, made of a frame, motors, a battery, a small flight computer and a payload. The payload is what the drone carries to do its job, such as a camera, a sensor or a parcel. On a military drone, the payload may be a weapon.
There are two main shapes. A rotor drone has propellers pointing upward, like a small helicopter, so it can hover over one spot. A fixed-wing drone has wings like a plane, so it cannot hover, but it flies farther on the same battery. A rotor drone suits a close job, such as inspecting a bridge. A fixed-wing drone suits a long job, such as surveying a pipeline.
The flight computer keeps the drone steady and follows commands. It reads sensors that measure the drone's tilt and acceleration, and it changes the speed of each motor many times a second to correct the drone's position. Commands come from a person at the ground control station, or from autonomy software that decides how to fly. The drone sends back its position and camera pictures over a radio link. Some radios are mesh and tactical radios. The camera may be an ordinary one, or a thermal imaging camera that sees heat. The pictures are used for drone surveying.
Say a police unit in Bhopal needs to watch a crowd at a festival. A rotor drone rises to 60 metres and hovers over the main road for about 25 minutes, which is what its battery allows. Then it lands, and its battery is swapped. Battery life is the main limit on a drone. Most small drones fly for only 20 to 40 minutes, because a battery big enough to last longer would make the drone too heavy to lift.
In India, drones are grouped by weight, from nano drones of up to 250 grams to large drones of more than 150 kilograms, and the heavier the drone, the more rules apply to it. Most drones must also be registered. One special kind of military drone is the loitering munition. Its level of autonomy decides how much a person controls it.