Cheeses harden because the aging process depletes moisture. The longer a cheese is aged, the harder it is. Hard cheeses may be aged or ripened by bacteria or mold.
Hard cheese is made with a slightly different process than soft cheese. Very soft cheeses such as cream cheese or cottage cheese are made using acid to coagulate the milk proteins. These cheeses are not aged at all.
Hard cheeses such as Parmesan or Asiago are ripened with bacteria and aged for over a year, creating a very hard, dry cheese. As the cheese ages, moisture evaporates and the outer skin of the cheese hardens into a rind. The longer the cheese is aged, the stronger the flavor.