The difference between Saltwater fishing and Freshwater fishing depends on where you do your fishing.   Lakes, streams and ponds are generally fresh water where a variety of fish may live, there are approximately 21,000 different species.  Approximately 2 % of the freshwater fish could live in saltwater. The rest of the species would loose  their weight and eventually die.  The freshwater fishing would be more favorable for the beginner.  Your equipment would be more on the lower end of the price scale to get started verses saltwater equipment.
     Saltwater fish live in the larger bodies of water such as Oceans, Bays, etc.  Fishing in saltwater requires heavier equipment and more upkeep, you can catch all sizes of fish in saltwater and the size you want to catch depends on your equipment, if you want to fish for Tuna, Marlin, or other large fish your equipment must be much heavier,  from your Rod to Reel to the larger size hooks and weights.
     Whatever your desires the milder degrees of freshwater or the more exciting experiences of saltwater fishing,  have fun and enjoy the relaxation of the sport.