On brushless DC motors, contrary to the normal DC motors, the magnet is rotating whereas the winding is fixed and part of the stator.
As indicated by its name, this type of motor is not using a mechanical commutation made by collector and brushes but it is using an electronic commutation.
In order to do that, the magnetic field is continuously monitored to ensure a correct timing of the commutation ( switching of the current in the windings through power transistor ).
Therefore, this type of motor can achieve very long life time and operate at very high speed.