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 Wall. I am 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."
Legend tells of a winter that lasted a generation, and of a vast and terrible darkness that fell across the land. It came to be known as the Long Night. In the midst of this darkness, the White Walkers emerged from the Far North. With their armies of the dead, they waged war against the living; laying waste to villages and holdfasts; leaving terror and destruction in their wake. After years of brutal conflict and unbearable loss, an alliance of the First Men and the Children of the Forest managed to drive the Walkers and their minions back into the frigid northern wastelands from whence they came.
To prevent another invasion, the First Men erected the Wall, a massive fortification 700 feet in height, stretching from the Frostfang Mountains, in the west, to the Bay of Seals, in the east. It was a structure unlike any ever built. Indeed, some maintain it could only have been completed with the aid of giants, or using the powerful magic of the ancient Children of the Forest.
Men were required to guard and maintain it, and thus the Night's Watch was born - a sworn brotherhood tasked with defending the realms of men against the dark forces that lay beyond. Upon taking his vows, a Brother of the Night's Watch serves for life. It is a life of hardship and great sacrifice, so the oath must not be taken lightly. The punishment for desertion is death.
The Night's Watch is divided into three vital branches: the Rangers, the Builders and the Stewards. While all Black Brothers are expected to take up steel should the need arise, the rangers are the true warriors of the Watch. Centuries come and gone, and although the white walkers have yet to return, another threat has emerged: barbarian tribes known as "wildlings". The Rangers are charged with defending the realm against these lawless savages. The Builders are carpenters, masons, miners and woodsmen, tasked with maintaining the Wall, as well as its various keeps, towers and structures, which have fallen into disrepair over the years. The Stewards serve as cooks, butchers and hunters. They tend to the horses and messenger ravens, sew clothing, gather firewood, and bring supplies up from the South.
The Night's Watch is a... diverse group. Proud volunteers from noble houses stand side by side with petty thieves conscripted from dungeons. Class distinctions are left behind, as are... past misdeeds. A man gets what he earns on the Wall; and even the lowliest guttersnipe can rise up in rank if he proves himself worthy.
For thousands of years, the Brothers of the Night's Watch have stood their lonely vigil. As the seasons changed, as brutal wars raged in the south, as dynasties rose and fell, the Night's Watch endured.
We are the swords in the darkness. We are the watchers on the Wall. We are the shields that guard the realms of men.
Source: http://gameofthrones.wikia.com