Holographic sights and red-dot sights are both variations of what is called a "reflex" sight. Basically, the idea is that unlike a scope, the reticle moves around the glass so it is always on the point of impact. The higher quality ones like the Eotech. Aimpoint, etc. have so little parallax as to be no-parallax for practical purposes.
A holographic sight projects a reticle on a screen using a laser. A "red dot" typically uses an LED diode to generate the light. The LED diode usually requires a tube-type construction.* The screen on the holographic sights can be bigger meaning you can track the point of impact over a wider area before the reticle disappears off the edges. They also play well with magnification (being a holograph). The downside is the laser is much more power hungry than an LED diode and the sights, while tough, are not as tough as some of their higher quality LED diode competitors.
*Some sights, like various cheap NcStar red dots, the C-More red dot, and the Trijicon Reflex attempt to do this without using a tube (or in the Reflex's case, without a diode either). The end result is usually a very dark "screen" in order to see the dot in daylight and the screen still tends to be about the same size as a tube sight anyway.
