Here is an excerpt from Chuck Hawks' Web Site generalizing on Red Dot sights, which is a type, not a brand.
I hope this overview will help.
Red dot sights
Electronic "red dot" sights have become increasing popular for handguns. Most of these look like short, fat, long eye relief telescopic sights, but have no magnification. They use battery powered electronics to project a red dot at the point of aim, centered in the optical tube, where the crosshairs of a conventional telescopic sight would appear. Red dot sights usually have conventional turrets with knobs for adjusting the windage and elevation of the sight, again similar to a conventional scope. Some red dot sights have dispensed with the tube altogether, and consist of a base with the optical and electronic systems necessary to project the red dot in a sort of "heads up" display. Most red dot sights come with rings, and mount by clamping to conventional Weaver-style scope bases.
The red dot sight's optics put everything seen through it (red dot and target alike) in the same optical plane, again just like a conventional scope. The long eye relief and zero magnification make it unnecessary to focus a red dot sight.
The intensity (brightness) of the red dot is controlled by a knob, usually in about 10 steps from very dim (for use in near total darkness), to bright enough for use in the brightest full sunlight conditions. The glowing red dot makes a very conspicuous aiming point.
Since small coin sized batteries power red dot sights, it makes sense to keep an extra set handy. They use very little power in operation, and a set of batteries usually lasts at least a couple of years in normal use. You must remember to turn the sight off when the day's shooting is over, however.
Red dot sights have proven in competition to be very fast sights to acquire, and since they have unity magnification, they can be used with both eyes open. They are a boon to pistol shooters, and have also seen use on "slug gun" type shotguns. I have three red dot sights for various pistols, and I like them very much, better than traditional iron sights or a conventional long eye relief scope for most handgun shooting.
Excerpt © 2002,2003 Chuck Hawks