HDR is surprisingly old
The first thing that comes to mind when we hear the term HDR are modern digital cameras taking multiple exposures and software that combines those images to colourful and funny looking images. As I have recently learned, HDR is much, much older than that!
HDR, high dynamic range, just means, that the image captured should have a wider range between the darkest and the brightest elements than the camera can natively capture or that the image contains a wider dynamic range than the output medium (paper, PC screen etc.) could reproduce natively. In the later case the images could be rendered as well and the process of compressing the dynamic range to the range of the output medium is called tone-mapping.
The french photographer Gustave Le Gray was the first one to capture two photos to combine them as an HDR image. He took one exposure of the sky and one of the foreground to create a natural looking landscape image (see example below). The resulting print, created in 1857, does not look too impressive to us today, but more than 150 years ago analog film had a very low dynamic range and was very sensitive to blue light. This normally led to a very bright white sky as the clouds and the blue sky were overexposed if the image was exposed for the foreground (or the foreground would just be underexposed/black otherwise). Le Gray shot one photo of the sky with a slow shutter speed and one of the foreground with a longer exposure and combined them while printing the image onto paper by masking out half of the image for each of the negatives. (To give some perspective: the photographic process was made public in 1839.)
Decreasing the exposure of the sky and increasing it of the foreground based on one or more digital images is still often done today in landscape photography.