A red dot
sight is an optical sight
that superimposes a tiny illuminated red dot over the point of impact
on the target you are aiming at, that is of course if you have properly zeroed
the sights. It comes in magnified and reflex sight
configurations, reflex meaning there is no magnification.
Red dot sights usually come as a small scope that attaches to the pistol via mounting rails
Its advantage over combat "three dot" handgun sights or regular iron sights is that there is only one focal plane that you need to focus on. The target and the red dot seem to be in the same focal plane and hence the sight picture is kept sharp. The effect, to the shooter, is similar to that of a laser pointer that are commonly used in tactical guns but there is no actual light that is emitted.
The most common brands are Tasco and Aim Point both of which are American products.
This nifty piece of hardware is quite expensive and is mostly common in IPSC open division race guns. It is not allowed in the more realistic IDPA competitions as it greatly adds to the bulk of a gun and makes it inappropriate for concealed carry.