The Night's Watch guards and maintains the Wall, and has castles all along its hundred league length. The great Wall was built by the First Men, long before the Andals came. It is seven hundred feet tall, and more than half that at its widest.
The Night's Watch are the men sworn to defend the realm against the men and creatures that inhabit the cold lands North of the Wall, where it is rumored that the Children of the Forest, giants, auroch sized snow bears, direwolves, mammoths, icy blue-eyed wights and creatures even more evil still exist.
But the Night's Watch has not faced these creatures for a very long time. The more real enemy that the Wall defends against are the Kings-beyond-the-Wall, the Kings of the wildlings, savages who live in the cold frozen wastes they Wall stands against. Many times in the last centuries have the wildlings tried to breach the wall into the Seven Kingdoms, only to be crushed by the Night's Watch or the forces of Winterfell on the other side.
The Night's Watch serves the realm, and nothing more. In times of rebellion they do honor to all Kings and obey none. The Sworn Brothers of the Nights Watch wear only black, take no wives, father no children and hold no other concerns beyond their duty. It is a commitment that lasts a lifetime and changes everything in the Night's Watch man's life. His old life and all that was in it is totally forgotten. All that remains is duty.
At present, the Night's Watch is facing what is undoubtedly one of its worst times; all of its nineteen castles bar three are abandoned for lack of men, their recruits are the dregs of society and the denizens of dungeons all over the kingdom, rapists, thieves, murderers and cripples. The ancient brotherhood seems to have fallen from the grace of its more glorious times and in the esteem of the people of Westeros.
This is the oath that binds a man to the Nights Watch: Night gathers, and now my watch begins. It shall not end until my death. I shall take no wife, hold no lands, father no children. I shall wear no crowns and win no glory. I shall live and die at my post. I am the sword in the darkness. I am the watcher on the walls. I am the fire that burns against the cold, the light that brings the dawn, the horn that wakes the sleepers, the shield that guards the realms of men. I pledge my life and honor to the Night's Watch, for this night and all the nights to come.