Pathfinder or as some people call it Dungeons and Dragons 3.75 is the spiritual successor to D&D third edition. Published by Paizo publishing who use to be the publishers of Dragon and Dungeon magazines. the game uses the D20 system and is about 90% compatible with prior 3rd edition material. this backwards compatibility is both good and bad. it is good so that 3e players did not have to start from scratch, bad in that the amount of bloat that 3e had is now bloat for pathfinder. also bad is that it prevents pathfinder from being to innovative with the rules as altering them too much wold result in loss of backwards compatibility.
the rules changes that were made are pretty good in my opinion. pathfinder eliminated dead levels in classes, so now every level gives you something, not always something cool but something. they simplified and streamlined combat maneuvers so no longer did the DM and table dread the phrase I am going to grapple the bad guy. there was some “re balancing” though that was not as effective as it could have been as over the years Paizo has added a ton of bloat and classes to the game.
Paizo’s business model is to release a rules supplement twice a year and two AP’s (adventure paths) with several small source books and guides for those Ap’s. it in my opinion be a better model that can help reduce the bloat if they instead focused on more AP’s and modules and less rules supplements. maybe do three Ap’s and two or three modules a year. as it is there are 36 classes not counting variants of those classes or prestige classes. there are 57 races stated up to be used as player races. there are way too many spells, and feats for me to even want to count. All of that by the way is just paizo published material, if you add wizards the number jump a lot. one nice thing though again bloat is that each class now offers archetypes that can change the core features of a class and allow for more variation.
now Paizo in my opinion does have some great adventure paths.and with some house ruling the game itself can be very fun. I would also suggest to anyone getting into Pathfiner to stay with core and slowly add supplements to the game to avoid the bloat as much as you can. another thing worth noting is that the paizo staff tend to be very active on their boards and are very open and friendly. they also support minority representation so if this is something that can help your choice in picking RPG’s know that they do support the reps. though it seems that when it comes to non man/woman couples they favor lesbians or trans* based couples as I do not recall that many two man gay couples.
if you like 3e d&d and are to afraid to try 5e then go with pathfinder it is D&D in all but name..the opposite of 4e actually which was D&D only in name. also note when 4e was out pathfinder actually outsold it. D&D was no longer the number one selling rpg. that is amazing and impressive that paizo could even for a short time dethrone the king.