Newborns and young infants need rear-facing car seats that are typically portable along with being easy to install or remove. The seats must be secured using the vehicle's seat belt. The seat itself typically has a harness that secures the infant in the seat.
As a child becomes older, there are seats that face the front. They typically feature a harness and some type of padded tray that comes down over the front of the child and snaps securely in place. Older youngsters typically need a simple booster seat that attaches to the vehicle seat and uses the over-the-shoulder seat belt to secure the child.