In photography, exposure is the amount of light per unit area reaching a photographic film or electronic image sensor. There is some different types of exposure, For example:
UNDER EXPOSED PHOTOGRAPHY Under exposed photography is the picture which has the small amount of light and it can be very dark or a bit dark. Also we can change the darkness to make it darker or whatever how dark we want it to be.
OVER EXPOSED PHOTOGRAPHY Over exposed photography is the complete opposite of the underexposed. An image that is brighter than it should be can be considered overexposed. When too much light is allowed during exposure, the result is an overly bright photograph.
PERFECT EXPOSURE Perfect exposure is the original exposure which is in the middle between underexposed and overexposed, and it's the normal exposure not dark and not bright.
About photography The photographers use the different exposures for a different photographes, For example sometimes they need to set under exposed for a scary movies.