Bulbs, corms, rhizomes and tubers store resources in different structures. Knowing which one a plant has helps with planting depth, division and seasonal care.
A true bulb has fleshy layers
An onion illustrates a true bulb: fleshy leaf bases surround a growing point above a short stem base. Lilies and daffodils also have bulbs, although their structures are not identical.
Corms and rhizomes are stems
A corm is a swollen underground stem, as in gladiolus. A rhizome grows along or beneath the surface and can carry buds and roots along its length. Some irises grow from rhizomes, while other irises grow from bulbs.
Tubers need another distinction
A potato is a stem tuber with buds commonly called eyes. Dahlias have swollen storage roots associated with the crown. These differences matter when dividing plants: a piece without a viable growing point may not produce a new shoot.
Let the structure guide the job
Garden catalogues often group these plants together as bulbs for convenience. Use the actual plant name for planting depth, division and storage instructions. Do not plant every storage organ deeply or expect all of them to need a dry winter rest.
Related reading: When to Plant Bulbs: Climate and Growth Cycle.