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The typical grain mixture for bourbon, known as the mash bill, is a minimum of 51% corn, with the remainder being rye, wheat, malted barley, or some combination of these. A mash bill that contains wheat instead of rye produces what is known as a wheated bourbon. The grain is ground and mixed with water.
As you said, whiskey may be made from either wheat, corn, rye or barley. American bourbon is predominantly corn, Canadian rye from rye, Scotch from barley & Irish from any of the aforementioned grains. Whiskey made from barley is tasteless, rye, bourbon impart good flavour.