A crib is a bed designed for an infant to use up until the age of three. The sides of a crib are high enough that a baby should not be able to fall or climb out easily. Cribs are designed for the mattress to sit high up, until the baby is able to sit up on his/her own, at which time for safety reasons the mattress must be lowered to prevent falls. Many cribs are designed with a side that will lower to help the caregivers while removing the baby from the crib.
History
Early cribs were beds that were draped with fabric to help keep the baby warm. As homes became better insulated and were able to stay warmer, the cribs became more popular to help keep babies safe. Cribs are now under strict regulations to ensure the safety of infants.