Generally someone from the rural southern US. Another name is hillbilly. There is a southern comedian and self-defined redneck who has made a living on "you might be a redneck if..." jokes. You can get an idea of what a redneck is from the jokes. Not overly urbane or sophisticated, not overly educated or well versed in litature. Of course it is all sterotype so no offense intended to "ya all" from the South. :D

Another one - you might be a redneck if you walk the dog and you both use the tree at the corner.

~I know nothing!~
Sergeant Schults from Hogan's Heros