The name echinoderm means “spiny skin”, so all of the animals in this group have some sort of protrusions on their exoskeletons. Those might be long spines, short spines, thorns, sharp stubs, or rounded bumps.
This group includes sea urchins (Class Echinoidea), which have the most obvious spines. But it also includes various types of sea stars, brittle stars or feather stars, a.k.a. crinoids (Classes Asteroidea, Ophiuroidea, and Crinoidea). All have a mouth centered underneath the body. They crawl around on the seabed to feed. Feather stars also use their spiny arms to strain food from the water and direct it to their mouths.
Sea cucumbers (Class Holothuroidea) look more like worms or slugs, although they belong to the same group. They have a mouth at one end and also use tentacles surrounding that mouth to gather food.