what is the difference between d&d and pathfinder?
The main difference is the rules. They are at their core the same thing, in fact Pathfinder was based on D&D 3.5 and was often called D&D 3.75. There really wasn't a ton of difference between 3.5 and Pathfinder, rules wise even. Now compared to D&D 5e, it's quite different rules wise.