Well, the brushes of course.
Yeah, but what does that mean?
The principle behind the internal working of both a brushless DC motor and a brushed DC motor are essentially the same. When the motor windings become energized, a temporary magnetic field is created that repels(and/or attracts) against permanent magnets. This force is converted into shaft rotation, which allows the motor to do work. As the shaft rotates, electric current is routed to different sets of windings, maintaining electromotive repulsion/attraction, forcing the rotor to continually turn.
Brushes inside electric motors are used to deliver current to the motor windings through commutator contacts. Brushless motors have none of these current carrying commutators. The field inside a brushless motor is switched via an amplifier triggered by a commutating device, such as an optical encoder.
Windings are on the rotor (Rotating part of motor) for brush motors and on the stator (stationary part of motor) for brushless motors.
By positioning the windings on the outside stationary portion of the Electric motor the need for brushes can be eliminated.
There are hints of Brush motors being first developed in the 1830s, by Michael Faraday
Brushed motor Advantages:
Simplified wiring: Brush motors can be wired directly to DC power and control can be a simple as a switch.
Brushed Motor Disadvantages:
Electrically noisy: The switching action of the commutators constantly creating and breaking inductive circuits creates a great deal of electrical and electromagnetic noise.
Lifespan: As they are in perpetual physical contact with the shaft, brushes and commutators wear out.
Brushless Motor Advantages:
Long lifespan: No brushes to wear out
low maintenance: No brushes to replace
Brushless Motor disadvantages:
High initial cost: Need for commutating device like an encoder and a drive or controller
Here is a cool video of a home made Brushless DC motor. Notice the amount of arcing in the reed switch as fields are switched. This is due to the release of latent energy stored in the windings as they are switched off.
The real take away on this video is that the magnets are on the rotating portion of the home built motor.
Brusheless motor vs brush motor efficiency:
Brushless motors are typically 85-90% efficient whereas brushed DC motors are around 75-80% efficient.
This difference in efficiency means that more of the total power used by the motor is being turned into rotational force and less is being lost as heat.