Classes make up a charatcer in many ways because it means if a player fights with a sword or a staff, whether a player heals in a war or goes around throwing poison. the differences are great. When creating a character pick carefully, it will mean the characters role in the game, as it is called a role playing game though.
Classes add abilities through the levels, quests, points, scrolls, books and many other things including trainers, boss hunts and raids.
The final thing is that classes make a role of a chaarcter, affect their skills, have certain advantages and disadvantages and can even get a player killed or not.