A region is a related collection of counties that may or may not span multiple states. The SRDC has defined 6 regions for study or comparison.
The Appalachia region extends from the state of New York south to Alabama and includes counties that surround the Appalachian mountains. The Appalachian Regional Commission (ARC) web site is a good starting point to learn about Appalachia.
There are 410 counties in the Appalachia region.