A bulb is an underground stem that stores food for the plant. Tulips are surrounded by a papery outer tunic . Lilies are covered with outer scales. Corms look like true bulbs on the outside except they are shorter and more flat. They have a solid starchy interior stem.
Crocuses are an example of corms.
Tubers are modified stems with starchy interiors but with no flat supporting plate or tunic. Both roots and shoots sprout form the same growth buds, called eyes. Potatoes are considered to be tubers. Rhizomes are thick underground stems. They grow in a horizontal direction, sprouting new sections as they grow.