Inside the DSLR
Sensor
The device inside of the camera which detects or measures a physical property and records, indicates, or otherwise responds to it. In the case of the camera, it detects light let in.
Shutter
In the camera - the shutter- lets you determine the amount of light let in and for how long, exposing the sensor in order to capture the image.
Mirror
The mirror in the camera is part of the prism system that permits the photographer to view through the lens and see exactly what will be captured.
Pentaprism
A five-sided prism that deviates light from any direction through an angle of 90°, typically used in single-lens reflex cameras between lens and viewfinder to present the image the right way round.