Maker's Mark bourbon is necessarily made from a primarily corn mash (all bourbons are required to be at least 50% corn among other requirements). But Maker's has an unusually high ratio of wheat as opposed to more typical rye or barley and is sometimes called a "wheated bourbon" as a result. The wheat is believed to create a much smoother flavor profile than some of its competitors.
The bourbon is made from a lake on the distillery grounds fed by limestone springs and said to be free of iron (which can turn whiskey green in the barrel). Their grain is purchased from small-farm cooperatives; they also are proud of using their own yeast. The whiskey is fermented in cypress wood tanks as opposed to stainless steel, again to reduce the taste of impurities in the final product.