What is the difference between the red-dot sight and the holographic sight?

The difference is that red-dot sights are cheaper to make, less accurate, and not as durable as holographic sights are.

Both use an LED (light-emitting diode) to create a reticle (target dot) imposed onto a clear surface, but holographic sights are usually more accurate at long distances and do not "change" position in the glass when looked through at a different angle, which the reticles of red-dot sights often do. The key to this difference is that red-dot sights refract the laser emitted by the LED only once, allowing room for error, whereas holographic sights refract the one beam from the LED multiple times from multiple angles so that each laser refraction will reach the same place every time and will appear to remain consistent in its position to the shooter's eye.