A generator will be a black cased with a large pulley on the end and is what would have came on your truck from the factory if it was built before 1963. By contrast an alternator will be in an aluminum housing, will be much shorter in length and much larger in diameter as compared to a generator.
The advantage of an alternator is that it is able to charge (produce current) at idle and low rpms whereas a generator is not. That allows you to have brighter headlights and easier starting. In addition most alternators have a built-in solid state regulator which makes the charging system much more reliable.
It is common practice to replace a generator with an alternator to gain the reliablity of a modern charging system. Thanks Randy
