If you are currently levelling a class and then decide that you no longer want to level it, you can drop it but lose all the EXP invested in that class, including the attribute points you get from those levels.

If you max out a class and then decide that you no longer want to have it take a spot, you can swap it out for a new one, and keep your levels that are invested in the old one. The new class still starts at level 0 though, and your stats will still take a hit from the sudden loss of your previous class levels.

You can swap at the Keystone. You can also, perhaps for the purposes of fulfilling certain requirements, reset your class's level to 0 at the Keystone.