There are generally three types of car seats, rear facing seat, convertible car seat, and booster. Rear facing seat usually is for babies who weigh less than 20 lbs. A convertible car seat can be used as a rear facing infant seat or a front facing toddler seat. It is used for kids who weigh between 20 and 40 lbs. As kids grow, a booster can be used. It is usually for kids who weigh between 30 and 80 lbs.