Prestige Classes
Basically, your primary class and secondary class are not shown once you reach level 20. Instead, once you reach level 20 (AND ASCEND), you're displayed as a generic title for your primary+secondary combination. This new prestige class you can level up even more, and gain access to new spells.
While you can still be a Monk/Warrior healer with some defensive stances... your "Judge" will gain access to smite spells powered by smiting prayers & divine favour.
Your Ranger/Ele can still be a trapper with Glyph of Concentration to keep from getting interrupted, but the "Druid" will gain access to powerful Earth-related spells.
Your Ranger/Mesmer would still be a DOT machine and get the nifty name of "Vagabond".
You get the picture.

By the way, it would also display your combo level starting at 0, so you'd be "Druid" until you gain a level and become "Druid1" (or Ranger/Ele21).
And, of course, you can change your secondary profession in the desert... becoming something else entirely if you do (though you can still change back, of course).