They are both types of dot sights that work a little differently from each other. The first link explains how a reflex sight works and the 2nd explains how a holographic sight work. A red dot will usually be a reflex sight, but a holographic sight can also be a red dot.
I tried to come up with a basic summary of both optics, but realized I would be largely just paraphrasing wikipedia. Too early in the morning for reading into and explaining how light, lasers, LED, holography, and parallax relate to these sights.