'Woodlouse' is the common name for creatures of the suborder Oniscidea. Oniscidea includes all of the isopods that have adapted to life on land. Most of us know them as small grey 'bugs' that live in rotting leaves and roll into a ball when you try to pick them up; these are woodlice of the genus Armadillidium, and are called by a variety of common names, such as pill bug, sowbug, roly poly, or simply woodlouse. While for the most part all woodlice look similar to the familiar pill bug, most cannot roll into a ball, and some have distinguishing features such as large splayed-out legs or interesting coloration (bright red or ghostly white). All woodlice, however, share a common core of features; many of the most obvious features, in fact, are shared not only by all woodlice, but also by all isopods. So let's start with some general information on isopods.
Isopods all have 14 legs, two sets of antenna, and a segmented exoskeleton. Behind the seven sets of legs there are a set of unassuming appendages that are used for locomotion in in the marine isopods, and for respiration in all isopods. Most isopods are quite small, but the aptly named giant isopod grows about as large as a horseshoe crab, looking like a giant trilobite, or for that matter, a giant woodlouse.
Woodlice are surprisingly like their marine cousins, having made only the minimal adjustments required for living on land. However, small as the changes may be, they are commonly underemphasized. For example, the common belief that woodlice have gills is incorrect; woodlice took the gill-like structures on their pleopods and added a set of rudimentary air sacs, resulting a structure known as a pseudotrachea. This structure works best when damp, so woodlice lurk in piles of rotting leaves and other protected lairs, preferring to come out only at night. But if need be, a woodlouse can leave these areas and travel through the world at large, including scorchingly dry environments.
Many species of marine isopods carry their young in fluid-filled brood sacs, from which the young do not emerge until they are nearly fully-formed miniature replicas of the adults (called manca). This is a very useful pre-adaptation to terrestrial life, and the woodlouse has continued this tradition of ovoviviparity. In woodlice the newly emerged young, although equipped with only six sets of legs rather than the full seven, are mobile enough to seek protection and dampness. If you ever get the chance to breed woodlice, watch for the sudden explosion of tiny little woodlicettes.
Speaking of breeding woodlice -- one of the adaptations that most animals make when moving onto land is to stop excreting the nitrogen waste resulting from protein metabolism as ammonia and dispose of it as urea or uric acid instead. This allows them to store the toxic ammonia within their bodies and conserve water. Woodlice went in a different direction. Woodlice dispose of ammonia by emitting it as gas, with the undeniable result that they stink.
Woodlice are almost always scavengers, eating rotting plant material or, in some cases, spoiled meat. This is largely because isopods need copper to build the hemocyanin in their blood (the equivalent to our hemoglobin), and the copper found in living plants is bound too tightly in stable organic compounds. Once the plants start to decompose the copper becomes more accessible, and the woodlice can digest it. If sufficient levels of rotting vegetable matter are not present, woodlice can also replenish their copper stores by eating their own feces. In order to avoid this eventuality, woodlice have evolved to store more copper in their bodies than can aquatic isopods (who generally absorb sufficient copper from the surrounding water).
There are between 4,000 and 5,000 known species of woodlice, so any description has to be general in nature. However, it is worth noting that there are species of woodlice that eat rotting meat, species that live in burrows in the hottest of deserts, species that live in a symbiotic relationship with ants (Platyarthrus hoffmannseggi, the Ant Woodlouse), and in hypersaline pools. Woodlice are the most populous and most widespread of terrestrial crustaceans.
When researching this writeup, I found many references to 'the common woodlouse'. As you might expect, the 'common' woodlouse is different depending on where you live. With approximately forty different families and thousands of species, the term 'common woodlouse' is essentially meaningless unless the writer specifies where exactly the woodlouse is common to. However, two of the most common woodlice are Oniscus asellus, the Shiny Woodlouse and Armadillidium vulgare, the Common Pillbug.
Recommended reading for practical woodlouse hunting.
While an truly unwieldy number of sources were used in composing this writeup, it would not be half as good as it is without the help of the book The Colonisation of Land: origins and adaptations of terrestrial animals By Colin Little.