How exactly do the sims 3 expansion packs work?

I saw that the new expansion pack for the sims 3 came out and i was wondering how it worked. I already have the sims 3 so does it add on to the game or is it a seperate game entirely?How exactly do the sims 3 expansion packs work?
It adds new content onto your game but you must use the new disk now instead of the original sims 3 disk. You must have the sims 3 to use it.How exactly do the sims 3 expansion packs work?
It adds on to your game.