Pretty self-explanatory. Hard plastic that work pretty well on carpet.
Carpet or Hard Floor Casters:
Softer (they have a polyurethane tread), so they get better traction on hard surfaces, and don't make loud noises when rolling around like the harder ones would.
A spring in the caster compresses when the user is in the chair. This disengages the braking mechanism, allowing the chair to roll. When the user leaves the chair, the spring expands and engages the brake so the chair won't roll easily.