What is the difference between an alternator and a generator?

Principle wise, both work on same electromagnetic induction, generator generates D.C while alternator A.C. In generator poles are on stator and winding on rotor, A.C generates in winding,which converts to D.C through commutator segments , connected at the ends of each winding, is collected through brushes, mounted on neutral axis.
In alternator winding is on stator and poles are on rotor, hence simple in construction, absence of commutator makes it more simple in construction, hence all high rating machines used for generating electricity, are alternator, are having less maintenance and manufacturing cost than generator, and are more economical mainly used in power generation..
It is better to call, it as D.C or A.C machines. As it gives electrical output when input is mechanical, and mechanical output (D.C motor and synchronous motor) when input is electrical. Use of D.C machines have increased tremendously, with the advent in electrical car( to run the car and to charge the battery) lot of research is going on in this field.