What is the difference between a fishing rod fixed for saltwater, amd one fixed for freshwater?
generally speaking a saltwater fishing rig would be a lot more stout than a freshwater rig, saltwater rigs would have heavy duty line guides and a heavy duty rod blank. a saltwater reel would have a lot of stainless steel and ceramic in it to be saltwater resistant, and have heavy duty gears and a lot of ball bearings- not to mention the size of the reel which would be considered huge compared to a normal freshwater reel. saltwater also dictates having a very good drag system, which is extremely important when you have a very large fish making a deep run.
there is also a difference between reel types for each. For example, freshwater fishing may be done with a spinner reel and angling rod or a fly reel and fly rod. A spinner reel spools the line perpendicular to the direction of the rod. The reel is mounted up to 24'' from the thick end of the rod. It has a geared mechanism which winds the spool. A fly reel spools the line in the same direction as the rod. this is mounted at the very end of the rod, behind the hand and has a heavy line with a leader line attached to the end.
A saltwater reel is generally constructed with brass, stainless steel, & nylon parts to resist corrosion from salt water. They are also beefed up in design to withstand the heavier loads from larger fish, baits & lines.
Ususly just the size and the line that is used.