Glory Destiny Online and Spirit Tales offer three races to choose from, the Gold Kirin, Maned Dragon and Lunar Fox tribes. Each race has two classes to choose from upon character creation. At level 40, each class can further promote into two subclasses.
Dungeons (aka. instances) are where class choices really effect how the game is played. They require certain roles to be filled and good cooperation between party members to clear. Below is a rough guideline on which classes can fulfil which roles.
A tank protects the party by taking all the attention of the enemies and the damage that comes with it. There are two classes which have the ability to tank, the Fighter and the Warrior, depending on their build. Fighters have an easy time forcing mobs to attack them and have more evasion. Warriors are bulkier with more HP and defence but take a little more practice to protect the party. Tanks may be hard to kill but they also have weaker attacks.
A DPS deals as much damage per second as possible. Without them, dungeons would take too long to finish. All classes can become a DPS with the right build. DPS classes usually have a mix of single target and AoE skills to deal with both mobs and bosses. Keep in mind that DPS generally can't take too many hits so dodge and/or have a tank take the damage.
A healer keeps the party alive by recovering health. This is probably the most important role in the party and a large responsibility. As of now, the Shaman class is much more desireable than the Sorcerer class when it comes to healing due to certain skill mechanics. Either of them can be a DPS depending on their build. Healers generally can't take or deal much damage but are vital in dungeons. Healers usually use wands to heal rather than staffs. Staffs are meant for DPS.
A hybrid is a mix of two roles, often with one favored over the other. A good example is the Sorcerer, which can opt to be a DPS with heals. Without sacrificing too much damage, they can level their heal to help out the main healer if necessary. Even if the party doesn't have a healer, the Sorcerer can bump the chances of success just enough to pass the dungeon. For all classes, one of the two roles has to be DPS but it doesn't have to be the focus (eg. a tank with more attack power).
In the end, it's up to the player's preference which class to pick, not what others demand in their parties. While there is no right or wrong way to play the game, dungeons can be unecessarily difficult if someone doesn't play their role properly. Choose a class, learn about it, and succeed with it. Most of all, have fun!