DC (direct current) motors require electrical power from a battery or a rectifier (a device that makes DC from AC - alternating current).
DC means the voltage applied to the motor has constant polarity (+ and -). The direction of rotation can simply be changed by changing voltage polarity (either at the stator or the rotor, but not both at the same time).
A DC motor roughly consists of a stationary part (the stator), featuring stationary magnets or field coils that create a static magnetic field, and a rotating part (the rotor), that features coils wound around the rotor axle. The opposition between the stator magnetic field and the rotor’s field produces rotation.
The speed of DC motors is directly proportional to the applied voltage, which influences the armature (rotor) current (more voltage = more current = increased speed).
DC motor torque does not depend on speed (unlike AC motors or car engines), so DC motors retain power even at low speeds. This is important especially for motors that must start under full load.
DC motors have brushes – parts that convey electricity to the armature while rotating. These are parts that wear out and must be checked regularly. “Brush” is a historical name that somehow stuck - today’s motors have carbon “brushes” that have a much longer life than the initial wire brushes.
There are DC motors that are brushless, but these are not true DC motors but actually AC motors that convert internally DC into AC (e.g. computer fans).