What is the difference between an alternating current generator and a direct current generator?

When electrical current flows through a circuit it moves by electrons in one molecule in a conductor [wire] "bumped" against the next molecule causing one of that next molecule's electrons to bump into the next molecule of the wire. This is true of both Alernating and Direct current. The difference is that in Direct Current circuits, the flow is always from, and in ONE direction. With Alternating Current, the flow direction changes, first in one direction, then in the opposite. In other words, back and forth. One set [one forward and one back] is called a cycle. When one hears about cycles in AC, that indicates how many cycles occur in one second, thus US AC 60 cycle current means that the current changes flow direction 60 times per second. In most of Europe, I think, AC current is set at 50 cycles. The AC cyclic rate is determined arbitrarily by the windings in the alternator, AND the RPM at which the alternator operates.