Millipedes and Centipedes are long with many legs. The main difference between the two groups is that millipedes appear to have two legs per segment, while centipedes have one. Millipedes are generally slower, and feed on decaying vegetation. Centipedes can be much faster, and often predate on other invertebrates. The latter can have painful bites, while the former have chemical defences. Millipedes tend to roll into a defensive ball when touched; centipedes shouldn't be touched!
A scientist who studies millipedes is called a diplopodologist!
Millipedes appear to have two legs per segment (Sinharaja, Sri Lanka).
When harassed, they tend to curl up (Sinharaja, Sri Lanka).
These millipedes are found in some parts of the old world tropics, from Madagascar, to India, through tropical Asia, and to Australia and New Zealand. They are often large, and roll into a tight hard ball.
'Giant Pill Millipede' (Poring Hot Springs, Malaysian Borneo).
When a giant pill millipede roll curls up, they create a tight ball immune to most predators, with the margins of the second and last dorsal plates fitting perfectly into one another.
This is the largest order of millipedes in the world, with over 3,500 species. They are called 'flat-backed', because their segments have a winged extension known as paranota. This group includes the species that defend themselves with hydrogen cyanide.
'Flat-backed Millipede' (Sepilok, Malaysian Borneo). This species looks like a centipede (and there are red centipedes in Asia to be wary of); but note the two legs per segment.