UAV's or drones have no human pilot on board. The flight is controlled autonomously in the vehicle or via remote control. Compared to traditional airplanes, they are cheaper in purchase and use, especially for small area applications. Flying below clouds, they are complementary to satellite imagery.

Allthough well known from their military operations, drones or UAV's are increasingly being used for civil applications such as surveying of pipelines, policing, firefighting, precision agriculture, climate research, archeology,...  There are a wide variety of UAV shapes, sizes, configurations, and characteristics. Degree of autonomy and Endurance are two important factores in UAV development. The most important restriction of UAV's is their need for very frequent communication, which is required to act in case an emergency takes place.  

The autonomy technology of UAV development falls under different categories: Sensor fusion (including automatic interpretation of ground imagery), Communications, Path planning (pre-programmed), Trajectory generation and regulation, Task allocation and scheduling and Cooperative tactics. 

The maximum flight duration of unmanned aerial vehicles varies widely between different models. Small devices which can be launched from the hand typically go for less weight and minimum payload. 

sources: http://en.wikipedia.org/wiki/Unmanned_aerial_vehicle

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