Infant: up to 20 lbs - rear facing Toddler: 20 - 40 lbs - forward facing
Most car seats have the information stamped right on the seat itself and instructions about how to use it (whether it be forward or rear facing) .. Also it is very important to use only valid, properly fitted and current car seats. Even baby car seats have an expiry date.
Backwards. One reason, if debris or glass flies from the front seat area to the back, during a collision, the high, hard back of the carrier helps protect the infant's head & body. It acts like a shield. There may be impact force issues as well, like whiplash. Babies necks aren't strong enough to handle that.