Westeros is a world roughly equivalent in size to South America. The northern lands of Westeros are less densely populated than the southern lands despite their roughly equivalent size, and much of the far North remains unmapped. This is due in large part to the extreme cold and the presence of hostiles known as wildlings that inhabit the region.

Westeros was originally divided into several independent kingdoms. But in the War of Conquest, these various kingdoms were united under the rule of House Targaryen into what is known as the Seven Kingdoms. House Targaryen divided the Seven Kingdoms into four principal regions - North, South, East, and West - and named one of its bannermen as the Wardens and Lord Protectors of each; House Stark the North, House Tyrell the South, House Arryn the East, and House Lannister the West. The five major cities of Westeros are, in order of size: King's Landing, Oldtown, Lannisport, Gulltown, and White Harbour.

Two other continents share the world in which Westeros resides: The continent of Sothoryos, about which little is known, lies to the South, and a vast, unnamed continent lies to the East, across the narrow sea. The closest foreign nations to Westeros are the Free Cities, a collection of independent city-states along the western edge of the eastern continent. The lands along the southern coastline of the eastern continent, collectively called the Lands of the Summer Sea, include Ghis and the ruins of Valyria, the former home of Westeros' Targaryen kings.

Tradition tells us that the known history of Westeros extends back for 12,000 years. Prior to this, little is known of the peoples and tribes that populated the continent. The children of the forest were perhaps the most advanced race, but their uncharted history is lost to myth and legend. Giants may have walked far and wide in these primeval days; other peoples may have vanished into the mists of time as well, such as those who left the Seastone Chair behind on the Iron Islands. Eventually, all would either disappear or fall to invaders from the east.

There are many renown strongholds throughout Westeros. These castles and fortresses frequently serve as the seats of power for the major houses and other large factions within A Game of Thrones.

Though steeped in several millennia of history, Westeros continues to have an undeveloped feeling compared to the rich traditions of the east. There are few major cities in the Seven Kingdoms, and much of the land maintains a rough and provincial feel. The vast uncharted lands serve to emphasise the hidden horrors of the North, providing the schemers of King’s Landing with a degree of isolation from the rest of the world.

There are many kinds of people found in Westeros; Rugged wildlings, honorable warriors, ignoble knights, unscrupulous mercenaries, power hungry nobles, venerated kings, mysterious assassins, and just about any other sort of character, from denizens of seedy seaside ports to the masters of lavishly adorned palaces. In Westeros, virtue and vice are not the province of any specific class. Rather, they exist freely in both ordinary and extraordinary characters in Westeros.