For me, flashbacks are basically memories brought about by something similar going on in the present. They can be caused by any of the senses or by reading/thinking something, "remembering" directly, etc. They are called "flash" because of their suddenly showing up in your thinking.

A hallucination feels a bit alien, is "out there" currently instead of in the past and one often isn't quite sure what's going on -- there's voices telling you things but you can't see the people talking or you see things other people aren't reacting to or are telling you they don't see, etc. There's an interaction element between you and whatever it is, you are not just "watching" and feeling like in a flashback.