Also known as a "BL motor". A brushless motor is an extremely efficient and often favored electric motor in the RC industry today. In a brushed motor (a motor with brushes), the brushes wear down after 2-3 dozens runs and have to be replaced. The comm also needs to be cut on a lathe every 3-4 dozen runs. With a brushless motor, none of this is required because a brushless motor does not have a comm and does not use brushes to spin the armature. It does not require any maintenance and will often out-live your entire car or truck.
A brushless motor requires a brushless controller - an electronic speed control (ESC) that has 3 motor wires - to control the speed of the motor according to the receivers output. If the brushless system is sensored, it will have another separate group of wires which monitor the armatures position to help prevent cogging. If it does not, it is considered to be sensorless.