Most RPGs have a set class system, but these games are different and have class systems that can be changed a bit.
Role-playing games have become one of the most popular types of games. This is because most RPGs are focused on the story and have a lot of fighting. Players love exploring a huge world as either a custom-made character or a pre-made one, interacting with interesting people, and fighting tough enemies who are a major pain in the side the whole time.
The class system is a big part of most hard RPGs. Players have to follow a set path that determines their skills, strengths, and flaws. Most of these classes are pretty rigid and drive players down certain paths, but other games do a great job of removing the limitations of this system by using a class to slowly guide players toward a certain style of play that they can always leave if they want to.