What is difference between an Alternator and a Generator?

In a generator, an armature is spun inside a magnetic field. The armature consists of several coils which generate electrical energy as they spin through the magnetic fields. "Brushes" (electrical devices to transfer the electrical energy from the moving armature) conduct the electrical energy to a wire. The strength of the electrical output can be controlled by varying the strength of the stationary magnetic field that is OUTSIDE the spinning armature. If that magnetic field is to be changed, it typically must be an electromagnetic.
An ALTERNATOR is exatly the opposite. The brushes make contact with two spinning rings. The rings are connected to coils, and when voltage is applied the armature (in the case of an alternator it is called a "rotor") becomes a spinning electromagnet. A stationary coil or set of coils, depending on the complexity of the armature, generates electrical current as the magnets pass by.
An alternator produces "alternating current" electricity which must be converted to "direct current" in the case of automobiles. The conversion is relatively simple, using what is called a "diode plate", which is as the name suggests, a collection of diodes.