A woodlouse is a crustacean

Crustacean

Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

 with a rigid, segmented, long exoskeleton

Exoskeleton

An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...

 and fourteen jointed limbs. Woodlice form the suborder Oniscidea within the order Isopoda

Isopoda

Isopods are an order of peracarid crustaceans, including familiar animals such as woodlice and pill bugs. The name Isopoda derives from the Greek roots and...

, with over 3,000 known species.

Woodlice in the genus Armadillidium

Armadillidium

Armadillidium is a genus of the small terrestrial crustacean known as the woodlouse. Armadillidium are also commonly known as pill woodlice, pill bugs or roly-poly, and are often confused with pill millipedes such as Glomeris marginata. They are characterised by their ability to roll into a ball...

can roll up into an almost perfect sphere as a defensive mechanism, hence some of the common names such as pill bug or roly-poly. Most woodlice, however, cannot do this.
Common name

Common name

A common name of a taxon or organism is a name in general use within a community; it is often contrasted with the scientific name for the same organism...

s for woodlice vary throughout the English

English language

English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

-speaking world.