Hillbilly is generally a person living in the mountains that have little street knowledge. They know lots about other things but not much about technology and such. Redneck on the other hand can reside anywhere and this is a definition from a web site I found. I think it sums it up well.
A redneck is usually typified in popular culture by a straight male with a beer belly that consumes cheap American beer such as Busch or Miller by the case (Pabst Blue Ribbon in more traditional settings) as well as Jack Daniel's. They are generally distrustful or dislike anyone not like them or the government. The stereotypical redneck lives in a trailer, and drives an old, large, beat-up pickup truck with a gun rack in the rear window. He generally wears a stained, sleeveless t-shirt, blue jeans, and a trucker hat.
Their hair is generally worn in the mullet style, and they favor long sideburns. Personal hygiene is a lost concept with the stereotypical redneck, and what teeth they have left generally show the complete anthology of the stages of dental caries. Their favorite activities include hunting, shooting at road signs and lights, professional wrestling, NASCAR, monster truck rallies, car engine repair, collecting junked cars and large appliances on their lawns, having way too many children and dogs, participating in domestic disturbances, and waiting around for their welfare checks. Country and Southern Rock bands such as Lynyrd Skynyrd figure in as their preferred genre of music.