There are three different types:

Convertible Seats: These convert from rear-facing to forward-facing. Try to buy a tall convertible seat, so that it will last a long time. This is because many children become too tall for their convertible seats--meaning their shoulders become even with the top shoulder harness slot--long before reaching the seat's weight limit and well before they are ready for a booster seat. Selecting a seat with a tall top shoulder strap slot will allow the seat to last longer.

Two convertible seats, placed side by side.

Combination Seats: These are sometimes called Booster Seats, but this name is misleading.  A Combination seat is a forward-facing seat with a 5 point harness that starts at 20-30 pounds and goes to 40-65 pounds with the harness. Then you remove the harness and use the seat as a booster until your child is 80 to 100 lbs. When selecting a combination seat, make sure that the top shoulder strap slot is going to be high enough for your child to grow with the seat--on many the top slot is no taller than on the convertible seat.