XP Farms are basically places where you can level easily from a certain mob or mobs that people prefer to farm for experience.
A recommended spot on Floor 2 for beginners is to the left of the spawn where there are 3 Boars in the area which is plentiful for them. But if you want something a little more challenging then to the right of the boss tower is a rock where the Dire Wolf and Wolf packs are mainly concentrated and keep you going.
Floor 4 has the area of Frozen Guardians where they have high XP give-out so they would be mainly targeted but are still hard for people who just get there, but people would rather go for the Floor 2 boss as it gives the same XP. But the Angry Snowmen are a must for anyone who just gets to the floor and the Icewhals would be farmed for their items.
Floor 5 Boss dungeon is a good XP farm for those who want to level up quickly and get Dusty Ore at the same time. People are kept busy due to the large amount of Centurian Defenders and their short spawning time, but that makes it extremely hard to survive for people who just came from Floor 4.