Maize made into grist is called grits when it is coarse, and corn meal when it is finely ground.
Wheat, oats, barley, and buckwheat are also ground and sifted into flour and farina.
There are some minor variations, such as "all's grist that comes to his mill", meaning that the person in question can make something positive out of anything that comes along.
A miller ground whatever grain was brought to him, and charged a portion of the final product for the service.
The term "gristmill" was once common in the United States and Britain to describe a small mill open to all comers.