Learn more at: http://www.faa.gov/passengers/fly_children/
A CRS is a hard-backed child safety seat that is approved by the government for use in both motor vehicles and aircraft. FAA controls the approval of some but not all CRSs. Additional information is available on the National Highway Traffic Safety Administration website. Not all car seats are approved for use in airplanes.
Make sure your CRS is government approved and has “This restraint is certified for use in motor vehicles and aircraft” printed on it. Otherwise, you may be asked to check the CRS as baggage.