You can read all the rules here for free.
House ruled versions of 3.5 classes. Monk is still terrible, druids got some well-deserved nerfs, bards did not, but on a whole, everyone is better. Thanks to splat books, monks do get better.
Paizo hates multiclassing, so they generally try to make it worth it to stay in a class from 1-20. If you were a fan of prestige classes, welp.
Races are improved. You always get a net positive to stats. Favored Classes got changed. Rather than allowing you to avoid an experience penalty for multiclassing, you either get an extra hit point or skill point for taking a level in the class.
Classes get archetypes, which are basically groups of alternate class features. You can choose to have an archetype when you enter the class. You don't have to choose and can just be vanilla.
You get more feats. Feats are generally worse than they were in 3.5.
Skill system was consolidated. This means that rogues don't have to choose between being a sneaky rogue or a social rogue. They can be both. Concentrate was gotten rid of. Now it's based on level.
Grapple, trip, and those various combat maneuvers were changed so they all work off the same bonus.
It's basically compatible with 3.5. If you have 3.5 material on hand, with some modification, you can still use it in a pathfinder game. Assuming whoever is GMing is fine with it.