A caster is a small wheel attached to the bottom of furniture or equipment that allows it to roll easily in any direction. Office chairs have casters so you can scoot around your desk without standing up. Hospital beds have casters so nurses can move patients from room to room. Shopping carts, suitcases, and even heavy pianos often have casters to make them easier to move.

What makes casters special is that they usually swivel, meaning they can rotate and change direction smoothly. This is different from regular wheels on a car or bike, which only roll forward and backward. When you push a cart with casters, you can change direction without lifting it or turning it around.

Good casters make a huge difference: they're the reason you can rearrange your bedroom furniture without asking three people for help, or why your teacher can easily move desks around the classroom to create different seating arrangements.