What's a bulb?
A flower bulb is really an underground storehouse and flower factory. There are many different types of 'bulb's, some being "true" bulbs like those of tulips or hyacinths.
Crocuses and gladioli, for example, are corms. They are generally flatter in shape than round, true bulbs.
Dahlias and begonias are tubers
Finally, there are rhizomatic plants such as bearded irises. Their rhizome (or root) has a swollen root that grows horizontally in the soil.
As opposed to bulbs and corns, tubers and roots have no protective tunic.