A small point of light is reflected off of a lense inside the sight. The shooter looks through the lense, and uses the dot as an aiming point. The lense is usually designed so that the sight is parallax free. The dot will be aligned with the same physical point regardless of the angle at which it is viewed. This makes shooting much faster, as there is no need for a shooter to precisely place his eye behind the sight, as with conventional iron sights.
Common light sources used in red dot sights include battery powered lights, fiber optic light collectors, and even tritium capsules. The color of the dot is usually red or amber for visibility against most backgrounds. Some dots are also visible when viewed through night vision devices. Newer red dot sights may use a chevron or triangular light instead of a dot for precision in aiming and range estimation.
Red dot sights are common in speed shooting sports such as IPSC, and are becoming increasingly common in military applications.