Little of the Outsiders or “Walkers” is known; in fact even researching about them can be considered a violation of a Law of Magic and can have you beheaded by a Warden. (So, you didn’t hear any of this from me!) They are not of our world, (in the sense that the Faerie, the Gods, and the Demons are still of our world.) They badly want to get in and if they do get in, ‘everything stops’.
In terms of power, they’re about as tough as the most powerful of the Demons or archangels, with the stranger and even more powerful entities akin to Lovecraftian Great Old Ones. It takes wizards centuries to develop defenses against them and magic apparently has little effect upon them. They can be defeated by, among others, the powers of the Knights of the Cross; however, Outsiders cannot actually be killed, only damaged sufficiently as to require long years to recover.
Outsiders live beyond the Gates, outside the Nevernever. Wizards are terrified of the Outsiders and of their power; however, as only mortal magic can summon an Outsider (neither vampires nor faeries can do it), the temptation to gain power can be irresistible, and the Seventh Law against Outsider summoning is unique among the Laws in that a single Senior Council member is assigned to enforce it full-time. Their internal organization is only sketchily understood, but they appear to have a caste system which includes empires and knights.