Botanically speaking a bulb is not always a bulb. Many times plants are sold as bulbs and unsuspecting consumers do not know the difference between a true bulb, a corm, a rhizome or a tuber.
Plants such as Oriental Lilies and Crinums come from bulbs which are a modified, shortened shoot that is enclosed by scale leaves. The food needed for this plant to grow and flower is stored inside the bulb. On Oriental Lilies the scales can be removed quite easily and another bulb will form from that piece of scale.
Gladioli and Freesias are examples of corms. A corm is defined as a swollen stem base where food is stored. Corms multiply and these plants need to be dug and divided. Dahlias and Begonias come from tubers. A tuber which is a modified swollen root is where this particular plant stores its food.
Iris and Canna come from rhizomes. A rhizome is a modified stem with scale leaves. Rhizomes grow at or below ground level. Iris is a prime example of the growing preferences of rhizomes. If you bury the Iris rhizome when you plant it you will find that instead of getting flowers you will end up with a mushy rhizome. That is because Iris needs to be planted on top of the ground where they are growing. The roots that grow from the rhizome will grow into the ground below and anchor the plant.
Understanding the difference between bulbs, corms, tubers and rhizomes can help you understand how to propagate your plants as well. The correct method of propagation is as important as knowing how to properly plant your flowers.