Whisky main material is grain. Starch in grain is converted to alcohol via sugar during making process.
It's germinated dried barley. Malt whisky uses only malt without any other grains but almost all grain whisky contain malt in mash together with other grains because only malt has enzymes for converting starch in grain to sugar.
Malt is dried and removed root just after germination to stop further growing up. Some malt is dried by peat and it makes peat flavor whisky.
There are 2 type barley, 2-raw and 6-raw. 2-raw barley Malt has good balance of starch and enzyme for malt whisky. 6-raw barely Malt has more enzyme than starch. It can covert not only starch in Malt but also starch in other grains to sugar. It is used for grain whisky sometime.
Rye malt is also used rarely for rye malt whisky.
Normally barley is converted to malt to use for whisky but Single pot still whiskey in Ireland uses non-malted barley partially together with malt in same mash (and other grain like oat rye and wheat traditionally)
American/Canadian rye whisky is made from mash which is more than 51% rye.
Canadian whisky is called rye whisky sometime but except for Canadian rye whisky, it is actually blended whisky of corn-main base whisky(Around 90%) and rye-main flavoring whisky(Around 10%).
American corn whisky is made from mash which is more than 80% corn. Bourbon require 51% instead of 80%.
Corn main mash enable sweat flavor and taste whisky.
But corn starch is hard structure so that some distillery boil milled corn before mashing
American wheat whisky is made from mash which is more than 51% wheat.
Due to loose PDO regulation in Japan, some spirits made from rice and malt aged in cask are sold as "Japanese rice whisky".
There are also a few barley and malt whisky in Japan.
In Addition of above, Oat is used for whisky traditionally. But some of new craft distilleries are challenging several grains like Farro, Foxtail millet, Buckwheat and Quinoa recently.
Milled Malt(Grist) Size
Milled malt is called "grist" Malt is milled to 3 different size before mashing.
Hask filters flower which cause clogging in mash tun.
It's main enzyme material for mashing
It cause clogging in mash tun. But grist always contain some flower and it is filtered by hask.