Unlike infant car seats, convertible seats are designed to last for several years, until your child is ready to move into a booster seat. Convertible car seats should be installed rear-facing until your child is at least 20 pounds and 1 year of age (or even longer, if you'd like - current recommendations suggest leaving it rear-facing until kids are up to 2 years), and then they can be turned around to face forward. Most convertible car seats can be installed either with a seat belt or by using the LATCH system.
When choosing a convertible car seat, here are some key things to consider: height and weight capacity, ease of installation, the size of the seat in relation to the size of your car, safety ratings and level of side-impact protection, overall comfort for your child, and ease of fabric cleanup/maintenance.