A stepper motor or a step motor or a stepping motor is a brushless DC motor, which works on a principle of electromagnetism, that rotates into discrete step angles and makes a complete rotation.
DC motor starts rotating just by applying DC power source to the terminals, but in the case of a stepper motor, it runs on a train of pulses (Square Wave pulses). Each pulse is making the shaft move through a fixed angle.
It is a brushless DC motor and as it is not having any feedback unlike servo motor so, it can be an open-loop controllable electronic device. It generally consists of the permanent magnet surrounded by a stator.
As you activate the windings step by step in a particular order and let the current flow through them, it magnetizes the stator creating electromagnetic poles respectively and that will cause particular angular rotation.