HDR (High Dynamic Range) is a method that aims to add more “dynamic range” to photographs, where dynamic range is the ratio of light to dark in a photograph. Instead of just taking one photo, HDR uses three photos, taken at different exposures.
Smartphones
With smartphones, three images (with different exposure) are taken and put together by the camera in-built software - to a new image. The brightest parts of the new image are taken from the underexposed one, and the darkest parts from the overexposed image. This method enhances the range of dark and bright parts of an image.
DSLR Digital Single-Lens Reflex
This type of cameras takes also three images, but saves them. The photographer may add these images by an external tool (on the computer) to the HDR image. Here the photographer has the control how the image should look like.