A holographic weapon sight is similar in nature to a red dot sight. Neither holographic sights nor red dot sights have eye relief or exit pupil. This means that unlike a typical rifle scope, there is no need for the eye to be placed within a certain distance to retain accuracy. That is where the similarities end and the differences begin to show. Unlike red dot sights, holographic sights use a laser to illuminate a hologram (an etched pattern of a reticle image) set between two pieces of glass. The result is a detailed illuminated reticle that may appear to be "floating" in the distance. This is especially useful in ranging. Again, eye relief and exit pupil do not apply, so the target is wherever the dot is pointing from the perspective of the shooter. This also holds true if the sight is damaged or partially blocked by an obstruction. Accuracy is always retained if you’re still able to see through the glass!
This does not mean that holographic sights are laser sights. The laser itself only illuminates the reticle and is not used directly for targeting purposes. Refer to the diagram below
It is also important to understand that the glass used to manufacture holographic sights is not typically curved, quite unlike a typical objective lens in a conventional magnifying optic. This means it will not reflect light as much as typical optics. Therefore, holographic sights do not require a killflash or Anti-Reflective Device (ARD).