At 75 you start some quests to add another class to your char.
The char starts at 40 and within limitations of RP you can chose what you want and start leveling again to become noblesse at 75/75.
Each class is independent of the others, so you can't go SH and then SE to buff yourself with empower (à la ff xi). You need to go to town everytime you want to switch classes and you lose your buffs / etc when you do.
You cannot sub warsmith nor overlord; elves can't subclass DE classes and vice-versa; some main classes may restrict your choices (like dagger can't sub dagger, etc).
What Orvieta said plus when you change to your subclass you will have the stats of the race of that class. So say, you are light elf and you subclass as destroyer. When you are on your subclass destroyer you will have the stats of an orc with the appearance of a light elf.
Tattoos are completely separate from main class to subclass as well.
Basically it is like starting a new character but on level 40 and with the appearance of a different race. Or the same race if you subclass a class of your own race and type.
You can also switch between main class and subclass at any time. You just need to visit the apropriate NPC for your race.