Nudibranchs move in the same way as snails and slugs by sliding along on a muscular foot. Nudibranchs are highly specialised carnivores that feed on animals that don't move much. Different species feed on different prey and some are totally dependant on a single species for their food. Some of the animals that nudibranchs eat include sponges, anemones, and bryozoans. All of these non moving prey species have various defences to protect them from being eaten that the nudibranchs can overcome.
Many nudibranchs can use the poisonous chemicals found in their prey in their own body defence.
People that have tasted nudibranchs think they are disgusting which is a good thing for the nudibranchs as not much will eat them. Some can even take the stinging cells from sea anemones and use them in their own defence. Most nudibranchs are very colourful which is a way of telling other creatures that their is something not very tasty about these slow moving creatures. This has good photos plus an FAQ which is reasonable and written quite simply.