What the difference between salt and fresh water rods?
The stick does not care where it is being used, but the components do. Make sure you choose a rod that is designed and outfitted to withstand corrosion. Guides and tiptops have to be made from non-corroding materials such as stainless steel or graphite, which makes saltwater rods a tad more expensive.
For the most part, not much.
As sheloves posted, the biggest differences are the use of non-corroding materials. St Croix rods has an excellent you tube video on how to choose a rod.
The other big difference is weight. A saltwater rod is generally made for larger fish. But that is not a 100% rule of thumb, especially when fishing for river or lake monsters.