Bigdaddy's Answer -- you know, once upon a time, a red dot sight meant exactly that .. a sight that projects an illuminated red dot as an aiming point onto an objective lens -- today though it is a "generic term" that most shooters use to describe a type of weapon sight that uses any illuminated color aiming dot or another shape for the reticle.

There are a few different types of red dot sights -- reflex, tube, prismatic, and holographic types. The difference in the types as I understand it is how the sight works to project the reticle. Reflex and tube sights use a reflective glass lens and an LED, prismatic sights use prisms and holographic sights use a laser.

Dot sights offer shooters a great advantage in speed and all imporatnt target acquisition. These types of optics are quicker and far easier than iron sights or magnified scopes. That is because they can reflect the reticle’s projection in parallel with the sight’s optical axis, ensuring the point of aim and point of impact always coincide. They are designed so the reticle is always in focus when pointed at your target. There is no aligning of sights and no adjusting for different distances. The dot stays in focus no matter the distance of the target.

If you want to read more about these sights please see our complete lineup at:

http://sightmark.com/hunting.php

And .. there are a goodly number of YouTube Video available to review:

https://www.youtube.com/results?search_query=sightmark+red+dot