Echinoderms usually have five appendages, but there are some exceptions. A lot of them have suckers on the ends of their feet that are used to capture and hold their prey, and also to hold onto rocks when there is a swift current. The differences between the two subtypes lies in how the arms connect to the central disk. Ophiuroids have arms that don't connect with each other. There is a distinct boundary between the arm and central disk.
Asteroids have arms that are connected to each other.
Also, it is harder to tell with asteroids where the central disk ends and where the arms begin. This is a picture of a Blue Linckia starfish. The starfish's skin looks very spiny if you examine it. If you look very closely, you will notice that there are different types of growth on the surface. Some bumps are used to absorb oxygen.