Floors are the different levels of Aincrad. and there are only one hundred. each of which getting smaller as you go to the next one. Floor one being the biggest with a whole 80 square kilometers radius.
Each floor has their own set of monsters, bosses, quests, and dungeons. as the higher the floor you get the more dangerous the monsters and bosses. and you can only get to the next floor by challenging the floor boss.
Here are the lists of floorsFloors 1-10