From my understanding (someone stop me if I mess this up).
A motor is an armature that spins because an electric current is applied to it.
A Generator is an armature that generates an electric current because it is spinning.
But they are physically almost the same.
However as the motor spins, it is still acting like a generator at the same time, usually the motor windings are set up in such a way that the current created by the spinning action works against the current driving the motor to spin. This means once the motor is spinning, it takes far less current to keep it turning (the apparent resistance is much higher). However if you jam the shaft of the motor to keep it from turning, the only resistance is what is inherent in the windings, and is much lower. This leads to overheating and can damage it.
They do this for motors that need to run a long time continuously to reduce the power they will consume.
Starter motors are wound slightly differently in that the current they generate from spinning works with the current driving the motor forward. This means they do not benefit from the lower current once spinning, although the spinning will help cool the motor from the air movement. This means they will have enormous torque but will overheat extremely quickly if they don't get moving. They also draw huge currents compared to similar sized motors.