We would say that the arthropod phylum is divided into several subphyla and classes (see our page on classification). Here are some of those subdivisions:
Sometimes centipedes and millipedes are lumped together as myriapods.
In the above list you can see that pill bugs are actually closer related to shrimp and crabs than to centipedes and millipedes, and that centipedes and millipedes, despite looking so similar superficially, are actually not too closely related.
Pill Bugs such as the ones shown at the left, take their name from the habit of rolling into a ball when bothered, protecting their soft underside. The pill bug at the far left is half rolled into such a ball. Pill bugs are about 0.6 inch long (1.4cm). Most crustaceans, such as lobsters and crabs, live in or near water, so pill bugs are unusual dry-land crustaceans. However, they still require moist habitats because their delicate gill-like breathing organs must be kept moist. Pill bugs are found under logs, stones, and in damp basements, and they feed mostly on decaying vegetation, so obviously they are not going to bite you. Pill bugs are often called sow bugs, but specialists like to reserve the name sow bug for some species that look a little like pill bugs, but can't roll into balls. Both pill bugs and sow bugs are sometimes known as woodlice.
You won't confuse pill bugs with centipedes or millipedes but, at least at first, you may have problems telling millipedes and centipedes apart. Here's how to distinguish them:
Centipedes are often called "hundred leggers," millipedes are called "thousand leggers," and pill bugs are sometimes called "sow bugs." At the top of this page you see a centipede fairly common in the US Southeast. It's a Scolopendromorpha, Scolopendra viridis. You know that it's a centipede because each body segment has one pair of legs (two legs) while millipede body segments each have two pairs (four legs). The Scolopendromorpha atop this page is about 3 inches long (7.5 cm), but they can grow larger. The larger ones can inflict a painful bite, and also can pinch with their last pair of legs. They dig burrows in which they rest, and females often coil around their eggs or young to protect them.
The photo at the right shows something most people don't know about centipedes: Some centipede species can produce silk. Mainly they use silk only during mating and capturing prey. However, the fellow at the right didn't seem to be doing either of those things, so maybe we've discovered something here!
The picture at the left shows a millipede. If you look closely you'll see that each segment is equipped with four legs, making it a millipede. I'm not sure about the genus and species of that millipede but I'm pretty sure the one shown below is Pachydesmus crassicutis, common in the southeastern USA. Both of these species are members of the Polydesmida order. Polydesmid species all lack eyes, nearly all have stink glands, and most have 20 body segments. On males, the first pair of legs on the 7th ring are gonopods, which are copulatory organs -- used during sex.
The flatish, orangish millipede shown on our Arthropod Index Page is a member of the Platydesmida order, and is a species of the genus Brachycybe. In that order, in contrast to the above Polydesmida, the bodies have 30 to 192 rings (not 20) and the 9th and 10th pairs of legs are modified as gonopods (not the 7th). Well, all this is being said just to show you that this business of centipedes, millipedes, and pill bugs is a whole new universe in itself...
Some interesting pages with centipede and millipede material can be found on the Web:
Though thousands of species of both centipedes and millipedes inhabit the earth there is no good, comprehensive, commonly available field guide for their identification. The Golden Nature Guide called Spiders and Their Kin illustrates about twenty of the most common species, so probably that's the backyard naturalist's best bet for beginners in North America.
You can review books about centipedes & millipedes available at Amazon.com in the US and the UK by clicking here.