A universal motor is essentially a series DC motor that can operate on both AC and DC power.
If you connect a series DC motor to an AC power source, it will still function because the armature and field windings are connected in series. Both the field current and the armature current reverse direction simultaneously, maintaining the same torque direction.
One of the most common types of motor around us (power tools, home appliances, etc.). Cheap but relatively low efficiency (30~60% efficiency).