Wikipedia
ArduPilot (also ArduPilotMega - APM) is an open source unmanned aerial vehicle (UAV) platform, able to control autonomous multicopters, fixed-wing aircraft, traditional helicopters, ground rovers and antenna trackers. Ardupilot is an award winning platform that won the 2012 and 2014 UAV Outback Challenge competitions . It was started in 2007 by members of the DIY Drones community. The first ArduPilot version supported only fixed wing aircraft and was based on a thermopile sensor, which relies on determining the location of the horizon relative to the aircraft by measuring the difference in temperature between the sky and the ground. Later, the system was improved to replace thermopiles with an Inertial Measurement Unit (IMU) using a combination of accelerometers, gyroscopes and magnetometers. Vehicle support was expanded to other vehicle types which led to the ArduCopter, ArduPlane and ArduRover subprojects.
Early versions of ArduPilot used the APM flight controller, an AVR CPU running the Arduino open-source programming language which explains the "Ardu" part of the project name, but today ArduPilot is almost entirely C++ and has evolved to run on a range of hardware platforms and operating system including the Pixhawk/PX4 line of autopilots, Parrot's Bebop2, Raspberry Pi based flight controllers like the NAVIO2 and ErleBrain and Qualcomm's SnapDragon.
The free software approach from Ardupilot is similar to that of the PX4/Pixhawk and Paparazzi Project, where low cost and availability enables its hobbyist use in small remotely piloted aircraft, such as micro air vehicles and miniature UAVs.