The flight control systems in our multi-rotors have a collection of sensors used to enable us to fly them.
3-Axis Gyro: The 3-axcess gyro detects rotation around all three axis of motion and enables the flight controller (FC) to stabilize the MR in pitch, roll, and yaw. It is this sensor that enables us to fly a mutlti-rotor. A few years ago, this was the only sensor many very inexpensive MRs had. A multi-rotor controlled by a FC with only a 3-axis gyro is flyable, but is very twitchy and is always going off in some direction requiring constant control inputs from the pilot.
3-Axis Accelerometer: The 3-axis accelerometer was the next sensor added to FC systems. It is this sensor that allows the MR to self level. Many inexpensive MRs sold today have only the gyro and accelerometer sensors.
Barometric Pressure Sensor: This sensor measures absolute air pressure and is used to calculate altitude change and allow for the altitude hold function in the FC. Without a barometric pressure sensor, the pilot must constantly adjust the throttle to hold a fixed altitude.
GPS Receiver and Compass. The GPS allows the FC to know its location and when combined with a 3-axis magnetic field sensor (compass) the FC can hold a fixed postion and navigate the aircraft to another specified position.
These are the sensors used in most 'full up' flight controller systems. DJI has added binocular digital cameras on the bottom and sides of their MR to enable visual detection of obstacles and the ground as well as precession landing.
Nick