The UAV - Unmanned Aerial Vehicle is a flying object that there is no person on-board. We used to name it a drone as well.
While saying drone, we usually visualize a quadcopter (i.e. DJI Phantom), but that is a very narrowed meaning. Drone or UAV is anything flying RC or autonomously, including other structures like i.e. helicopter and fixed-wing ones: plane, soarer, flying wings, also many hybrid constructions.

While the drone operator (UAVO) can be a person that manually controls the drone using a remote connection, it is also possible that they may create a flight plan (mission) and upload it to the drone's flight controller to let it follow it or even to let it perform independent decisions on how and where to fly. Those are autonomous flights. In any case, there should be a human person that even if the flight is autonomous, may monitor mission progress and take manual control in case of an emergency.

From this point of view, we consider flight modes of the UAV that are one of the following:

• Remote Control - operator “drives” a drone.
• Mixed-operator “drives” but automation of the flight control may interact with the drone, i.e. introduce collision avoidance, smoothen tilts, and so on.
• Autonomous - where the drone follows the flight plan (mission).
  • Mission plan can also be modified ad-hoc, using onboard decision systems or communication (but human independent, or with the limited impact of the human operator).

From the legal point of view, the relation between the drone and its operator, regardless of the flight mode, as mentioned before, is an operation mode and can be classified as one of the following:

• VLOS - flights within Visual Line of Sight Range: operator must be able to see the drone using unaided eye (well, you can really wear glasses if you need one on a regular basis). The operator must be able to operate the drone in RC mode (even if the mission is autonomous just in case of emergency), so seeing your drone as a single point some 1km away from you is believed not to be a VLOS, unless you have an eagle eye. Of course, if you need to have a quick peek at the video stream or telemetry display to see flight parameters, it is all valid but please note, when your drone is far away, and you lost its sight, it is not easy to find it back again so you may get into the trouble.
• BVLOS - flights Beyond Visual Line of Sight: operator not necessarily needs to see the drone, but one should be still aware of where it is now, using onboard and ground station instrumentation (usually some telemetry, video link, mission control software, and so on). BVLOS is both situations when your drone is too far away to be seen (even if not hidden behind another object) or is close enough, but hidden, i.e. around the corner of the building.
• FPV - First Person View flight, when UAVO wears special glasses (or uses a ground display) that presents live transmission from the drone, as seen by the “virtual pilot sitting in the drone's cockpit”. That is the most common case in drone racing.

In any case, nowadays, the drone is not just only a piece of hardware but the whole ecosystem: growing as there is a demand for more and more complex tasks. It includes hardware, software, humans, and perhaps in the nearest feature, artificial operators, communication infrastructure, and procedures. In the following sections, we go into the details of each of those components.