Millipedes are slow-moving, and have two pairs of legs per segment, while centipedes, which are carnivorous, ony have one and usually (not always) move faster. Millipedes will curl up into a spiral when disturbed, and stay put, which centipedes won't. They feed on dead plant material, and can be a minor nuisance. I don't kill them myself, as they must do some good getting rid of rotting dead stuff.
