Caladiums and tuberous begonias are tubers that appear more like corms. The primary difference is that a tuber can sprout roots from the sides and top, just like that potato you found behind the microwave the other day. Other tubers, like the anemones, are woody. They share the rooting characteristics of true tubers. Depending on whom you ask, tuberous roots are either lumped in with tubers, or are a separate variety.
Dahlias have tuberous roots, as do the sweet potatoes.
Typically, tuberous roots have a pointed bud at the top and root more toward the bottom. The final version of bulbs is not a bulb or root at all. Rhizomes are really thickened stems that run horizontally. Rhizomes grow leaves and flowers from the top and, rather expectedly, roots from the bottom. Common rhizomes include the calla lilies, cannas and bearded irises.