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.
Tubers are outgrowths from internodes on underground stems.
They are characterized by having growth buds of 'eyes'. A white potato is an example of a tuber. Tuberous roots are actually roots that are specialized to store nutrients. They grow out of the stem base and they are characterized by having no growth buds or 'eyes'. Dahlia 'bulbs' are really tuberous roots.