There are 37 species of native or naturalised woodlice in Britain and Ireland, and they are one of the most common and widespread animals which although familiar are mostly ignored, unless one wanders into a dwelling. They are quite a diverse group, and are members of the class Malacostraca, which also includes crabs, shrimps and lobsters. There are around 10,000 species of Isopoda worldwide, most of which are marine.
Woodlice are often considered as pests in gardens and greenhouses, and when they enter houses. They are completly harmless, and although they may nibble on the odd tender seedling, they play a very important roll in recyling and the enrichment of soil in ecosystems, especially woodland. They feed mainly on dead plant matter like leaf-litter and rotting wood, which in digestion is turned into nutrients, and with the help of fungi helps speed up decomposition and fertilises the earth. Woodlice are also a good food source for birds, frogs, toads, shrews, spiders, centipedes and other predaceous invertebrates.
Many thanks to Warren Maguire and Steve Gregory for their expert assistance in identifying and reconfirming species.
For further reference see the links and literature below:
An AIDGAP publication A Key To The Woodlice of Britain And Ireland by Stephen Hopkin is also a very good reference guide.