Children's car seats are divided into groups, based on the weight of the child. Remember that not only the age, but also the weight of your child is important. All car seats must come with the SABS standard of approval. And where possible, rather invest in a new car seat, and not a second hand one. A used car seat might not come with all its components intact.
Infant - Birth to a Year
The best kind of car seat for an infant is a rear facing car seat. Because of its snug fit, a rear facing car seat is the best. Its also light and convenient which means when baby takes a nap, you don't have to wake him or her up. You can simply carry him or her into the house in their baby seat.You can also purchase a convertible car seat which converts the rear facing into a forward facing seat once baby is bit bigger and older. (Its also more bang for your buck because it means you don't have to buy two car seats to accommodate your baby's growth) Convertible car seats usually cater for sizes 0 - 13 kilograms.
When to switch?
Your baby is safest in a rear facing chair for up to a year before you change to the toddler chair, that way if there is an accident then the energy of the impact is absorbed and distributed by the seat shell and car seat belts.
Whether you're purchasing an infant car seat or a convertible car seat, look for a model that has a 5-point harness with two shoulder straps, two waist straps, and one strap between the legs that meet in the middle. Also ensure it has side-impact protection or extra foam or air pads at the side of baby's head. Toddler seats will hold a child from age 1 to about 4 years old, or 9-30 kilograms depending on the size of the chair. Thereafter you may simply use a booster seat, for height but buckle up with regular car seats at all times.
Babies and messes go hand in hand, but a surprising number of car seats come with covers you can't take off. A detachable, machine-washable cover makes cleanup much easier.
Here is a great info graphic from the US Insurance company 21st Century