So while I'm sure there are thousands of different kinds of milli- amd centipedes, in general:

• Millipedes have shorter legs
• Millipedes are slow. Like, slow enough to see them move unlike centipedes, creepy bastards
• Millipedes are vegetarians, and while crushing them can be bad news, they don't bite
• Millipedes look kinda like long pill-bugs, whereas centipedes look like hell had a nightmare and it was given flesh and form