above is a motor animation.  The shaded area represents exposed copper.  when the wire comes in contact with the copper, a current flows through the loop of wire.  in the presence of the magnetic field (B), a torque is created on the loop of wire.  This torque causes the loop to rotate.  if the current were allowed to flow continuously, the torque would act to stop the rotation.  This is the point at which the copper no longer makes contact with the wire, and the current ceases to flow.  the momentum of the loop carries it around until the copper makes contact again.  This causes a torque in the same direction as before, and the motor accelerates until opposing forces reach an equilibrium, and the motor runs at a constant speed.

video of a motor based on this simple design:

dc motor.wmv - ~ 7.6 Mb