There are a number of plants that are called, 'bulbs'. True bulbs are actually large buds with a small stem at the lower end surrounded by numerous fleshy leaves called 'scales'. An onion is an example of a true bulb. The roots come out of a solid, bottom basal plate. The bulb-like plants include corms and cormels, fleshy rhizomes, tubers and tuberous roots.
Corms are bulb shaped, underground stem bases with a solid, bottom basal plate.
Corms look like bulbs superficially but differ in that they are composed almost entirely of solid, Gladiolus 'bulbs' are really corms. Cormels are undeveloped corms. Rhizomoes are horizontal stems that grow underground just below the soil surface. They look like roots but they are characterized by having scale-like leaves and axillary buds at the growth nodes. Bearded iris 'bulbs' are really rhizomes.