Primary and secondary classes explained

Here follows an explanation of the dual class system of Runes of Magic.
When you create your character you will choose race and class.
When your Primary class reaches level 10 you can choose your secondary class in the Varanas class hall.
When both your classes reaches level 20 your can choose your third class, also in the class hall in Varanas.
You can switch between all classes whenever you want, you just need to visit the right NPC to do so.
You can do it in Logar, the same NPC you visit to enter your house, and in the class hall in Varanas of course.
You can also do it in other cities and camps along the way of your adventure.

You have three classes but can only use two at a time, primary and secondary class.

You can only level up your primary class, so you need to switch classes to level them all up.
Your secondary class level can't exceed your primary class level but you will never lose any levels, your secondary class will only be temporarily capped at the same level as your primary level until you level it up.

All classes have different skills, some of them can only be used as primary and and some both as primary and secondary.
It's good to have at least one class with ranged weapons so that you can select enemies to attack and don't have to run into a big crowd trying to kill them all at the same time.

Read more about choosing classes here.