A device used to crush grain. Multiple types exist, each with each own characteristics.
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.
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.
Can mill all grains but is normally used for corn. Provides an inconsistent grind size and destroys husks. Has some adjustablity. Most common brands are Corona and Victoria.
Unless you're stuck in the 1800's or you're a historical reenactor you won't be using a Stone Mill. It's the old fashion type where big stones roll on top of each other.