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.
So..there are thousands of rods out there and for good reason. Most to all rods are going to be some kind of glass, graphite, composite layup. Application widens the spectrum of rods allowing them differing tolerances with action, pulling power, and sensitivity. Between the two worlds, there exists everything on the spectrum. Been building rods for going on 16 years, and... have I wrapped silicone carbide or ceramic guides on a freshwater blank? sure I have! 😎Research the company or brands you are interested in, and try and figure their main focuses. I wouldn't look into Calstar or Seeker rods looking for finesse bait drop shot performance. On the other end of the spectrum, I wouldn't be looking for 10' Jigsticks looking in a Lews catalog.