George has a great page of DC motor tips, he writes -
A small DC motor generates torque by creating an interaction between a fixed and rotating magnet field. The fixed field is supplied by high energy permanent magnets. The rotating field is created by passing a DC current through several different windings on the armature (rotating part) and timing which winding is powered through a device called a commutator. Power is applied to the armature by brushes which ride on the commutator.
To understand how a motor responds to load, the motor itself is modeled by dividing it up into three major components. These components are the ideal motor, a back-EMF generator and parasitic resistance. These parts are really not physically separable, but for modeling purposes this is convenient.
Small DC motor tips - [Link]