File Powder is a spicy herb made from the ground leaves of the sassafras tree. It is used in Cajun cooking when making gumbos as an herb and as a thickening agent.
I just want to add that It should only be stirred in at the end of cooking. If allowed to boil, filé powder will cause a liquid to become stringy and unappetizing.
drbabs has it exactly right. Although I would add that people unfamiliar with Louisiana mix up creole and cajun cooking and this is more the former than the latter---although it is used in the cajun part of the state.