What exactly is a DC motor?
Doesn't it also run on AC?
Generally using DC (or Permanent magnets) to supply a fixed polarity to the field coils, and brushes to provide alternating current/polarity to the armature.
Or visa-versa, providing fixed polarity to armature, and some switching technology to vary the polarity of the field coils.
A DC motor is any motor that does not require an AC supply. They generally include a commutator which maintains the correct relationship between the armature and stator fields so that a torque is always tending to rotate the armature in a particular direction.
If the stator field is produced by a permanent magnet, the motor can only operate on DC, but if the stator field is produced electromagnetically from the supply current, the motor will run on AC or DC, and it is, essentially, frequency independent, unlike most other AC motors who's rotational speeds are frequency dependent.