The starfish, or sea star, is not a fish - starfish have no skeleton. They are related to sea urchins. As they have no skeleton and hence no legs, they don't move around by walking. They move by pumping water around through tubes on the base of their arms. Not all starfish have five arms, some have more.
Starfish eat sea creatures with shells that move slower than it does. Starfish have two stomachs, and one of them can be put outside the starfish's body. So they eat clams and other shellfish by forcing them open and putting their stomach inside the clam's shell.
Some starfish can regrow arms if they are cut off. Some starfish can grow from the cut-off arm as long as some of the central portion of the starfish is still attached.
Starfish send their eggs out to drift into the open sea where they hatch into small swimming larva. When the larva are old enough, they sink to the bottom of the sea and turn into adult starfish.