Potatoes are often referred to as root crops. Potatoes are thought to be roots because they grow underground. However, potatoes are not roots. Instead, potatoes are specialized stems that shoot off from the root. These offshoots are called tubers. The potato tuber serves as a massive storage reserve for a range of macromolecules, such as starch and protein. These storage reserves serve as energy warehouses for the plant in times of need. However, they have becomes a staple in the diets of many consumers (organisms that eat other things in order to survive) as well. Potatoes contain vitamins such as potassium that add nutrition to the diet of consumers.
Other examples of tubers are sweet potatoes, cassava, and Dahlia.
The link below provides additional information about tubers.