The stator is the fixed part of a rotating machine. The rotating part of the machine, or rotor normally rotates inside the stator. Examples of such machines are pumps and motors.
The stator provides the external magnetic field and remains stationary while it surrounds the armature and contributes to the torque. In simple DC motors the stator is made with permanent magnets. Stators can also be made from electromagnets. Electromagnets are more commonly used and have many advantages. By reversing the current through the electromagnet, you can reverse the polarity of it thus reversing the direction of the motors rotation. The strength can also be changed, which changes the speed of the coil rotation. It can also be turned on or off meaning that it does not always remain magnetized. Most stators are given a curved shape. This makes the magnetic field curved and increases the time when the armature is parallel to the magnetic field, giving it maximum torque.