AC generators can be broadly referred to any type of electricity generator that generates electrical energy in Alternating Current (AC) form. So, in a way, yes, an Alternator is a type of AC generator. Just so, an Induction generator is a type of AC generator too.
A more appropriate term for an alternator is - AC synchronous generators.
More specifically, in an alternator, the excitation winding (which produces magnetic field) is mounted on the Rotor shaft and the armature winding (which generates electrical current) is mounted on the Stator (stationary part of machine which surrounds the rotor). While in most of the other types of electrical generators, including DC generators, the excitation winding is stationary and the armature winding is mounted on the rotor shaft.
So basically, you may distinguish it like - the magnetic field is stationary in generators, whereas it is rotating in alternators. Or the other way - the armature winding is rotating in generators, whereas it is stationary in case of alternators.
So why the armature winding is stationary in an alternator?
- At high voltages, it is easier to insulate the stationary armature winding, which may be as high as 11 kV or even more in some cases.
- The generated high voltage output can be directly taken out from the stationary armature. Whereas for a rotary armature, there will be large brush contact drop at higher voltages, also the sparking at the brush surface will be a problem to look after
- If the field exciter winding is placed in the rotor, low voltage DC can be transferred safely to the exciter winding via slip-rings.
- The armature winding can be braced well, to prevent deformation caused by high centrifugal force if it was in the rotor.
Take it like this, all alternators are AC generators but NOT all AC generators can be alternators.
In general terms you can read more about Difference between Generators and Alternators here.
