Sea stars are quite a big group in the phylum Echinoderms (spiky skin). The main ones are split into two different classes: Asteroidea, which are the ones commonly called sea stars or star fish, and Ophiuroidea, commonly called brittle stars. They differ in many aspects, the shape and the means of locomotion being the most evident ones.
The central part of a brittle star is disk shaped, relatively small compared to the diameter of the star and the arms (in almost all the species there are five) are very thin, very flexible and manoeuvrable. The simple nervous system of these organisms coordinates the movements of the arms so the sea star can basically 'walk' on the sea bed by lifting the arms alternately, which you can see with your bare eyes. Some species live on the sea bed whilst others live in crevices or on coral. Most of these stars are herbivore: they consume plants, algae and organic debris along the way.
Most of these stars are carnivore and over evolutionary time they have developed a very peculiar way of eating. Most of their diet consists of bivalves (i.e. shells like mussels and clams). They will surround the shell with their arms, attach the suckers of the pods on the sides of the shell and slightly open it. They will then insert their stomach inside the shell and start to digest the soft part.
All the stars we have talked about here have their mouth on the lower side and the anus on the upper one; they have a sort of eye at the end of every arm that acts has a photoreceptor; they are mostly hermaphrodites (have both sexes) and they have a pelagic life at larval stage so they can disperse better.
There are three other groups in the same phylum: sea cucumbers (Holotudoidea), sea urchins (Echinoidea) and sea lilies (Crinoidea, i.e. feather stars). They all have, of course, similar traits and characteristics but differ in some of them.
The phylum Echinoderms has an embryonic development that is extremely close to ours. The spilling of the embryo is called undeterministic, which means that in the first stage of cell splitting, no cell has a specific duty assigned. Later in the development some cells will become skin, some will became liver, etc. So if you remove one cell in the very early stage of development, the organism will grow up to a fully functional one. Other organisms have deterministic development, so the cells will have their duty assigned immediately and if one is removed the organism might not survive to full development.