What is the difference between crabs and hermit crabs?

Most crabs have shells, but the hermit crab is a type of crab that doesn't. Instead, they adopt discarded mollusc shells such as whelks and periwinkles, and use their shell.

The term crab is often applied to several different groups of short (nose to tail) decapod crustaceans with thick exoskeletons, but only members of the Brachyura are true crabs. Other taxa, such as hermit crabs, porcelain crabs and king crabs are, despite superficial similarities, not crabs at all; rather, they belong to the Anomura and can be distinguished from true crabs by counting the legs. In Anomura, the last pair of pereiopods (walking legs) is hidden inside the carapace, so only four pairs are visible (counting the claws), whereas uninjured true crabs always have five visible pairs.

https://en.wikipedia.org/wiki/Crab