Grain mill
A device used to crush grain. Multiple types exist, each with each own characteristics.
Roller Mill
The most common for beer brewing. Grains are passed through 2 or more rollers to crush grains without disturbing the husks. Most are adjustable down to a small kernel. Good for most grains except corn. Corn can be too tough for the mill to crush.
Hammer Mill
Used commercially for grains. Unlike the Roller Mill the hammer mill will damage the husks. Uses screens for size adjustment. Kernel size can get be as small as flour. Can handle all grains including corn. Hammer Mill Animation Hammer Mill vs Roller Mill.
Burr Mill
Can mill all grains but is normally used for corn. Provides an inconsistent grind size. Has some adjustablity. Most common brands are Corona and Victoria.