Woodlice are crustaceans- they're basically land-shrimp. They belong to Order Isopoda, a group of 10,000 or so species, half of which are land-dwelling woodlice, half of which still live in the water. Their marine relatives include some that can grow to 50cm (2.5 feet) long, and also some rather disturbing parasites such as the "tongue-eating louse."
Being crustaceans, woodlice have larvae that need to hatch out in water. They solve this problem by having little pouches full of salt water, in which the eggs hatch and the larvae spend the first few days of their lives. So they're kind of like tiny, many-legged marsupials.
Pillbugs are a family of woodlice that can roll up into a neat, shiny black ball for defence. As a kid, I thought that all woodlice could curl up, and didn't understand why I never saw them do it. I saw the rolley-up kind for the first time a couple of years ago- one of the buildings I work in seems to be in pillbug territory. Pillbugs can be confused with pill millipedes, which are just very short, wide millipedes that can also roll into a ball. The main visible difference is that the millipedes have more legs. A nice bit of convergent evolution- the two different groups are about as far apart as you can get on the arthropod family tree.
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