Louisiana is unique in that its regional governments above towns are parishes instead of the traditional county system found in the other 49 states. That’s not to be confused with the other common meaning of parish, e.g. a church community.
Just substitute the word PARISH for COUNTY and you are good to go in Louisiana research.
Here are some free sites with which you can begin your search:
FamilySearch Research Wiki – Louisiana
Louisiana Genealogy – Access Genealogy
RAOGK – State of Louisiana Records Guide
Free Louisiana Genealogy Search Engines – The Ancestor Hunt
Louisiana Digital Library
Louisiana State University – University Archives Digital Collections
Xavier University of Louisiana – Digital Library
Chronicling America – digitized historical Louisiana newspapers online
Linkpendium – hundreds of links to Louisiana parishes
Central Louisiana Genealogical Society
Terrebonne Genealogical Society