You get one stance (uses endurance, based on melee-ish classes) and one invocation (modifies spell casting). You can switch them around almost at any time and they have 6 second independent cooldown.
Getting your ass beat? Defensive stance for 50% less attack damage taken and 20% magic damage taken. Not under duress? Offensive stance for double attack damage.
Need to get a quick heal off? Inversion for giga cast speed, but your spells have a long cooldown. Pulling a boss and REALLY need slow to land? Overchannel for a huge -resist check on your spells. Want to play shadowpriest? Stack wisdom classes and use Divine Invocation to make EVERY spell you cast heal your lowest hp groupmate with the mana cost as healing. Nuke away and little Timmy is getting healed.
In solo dungeon/oh shit situations you switch between these as the circumstances demand. Most of the time you're crusing around in your giga damage setup until you pull 5 too many mobs or something, but in BIG shitballs or against GIGA bosses, you're actually PLAYING the game rather than auto attacking and hoping you win.
They basically gave you button to click that make your character do different stuff and they're not 15 minute CD's. A truly new EQ thought.