The Known World is composed of four continents (Westeros, Essos, Sothoros and Ulthos), and a number of many smaller islands. None of the four major landmasses have been completely mapped.
Located in the far west of the known world, Westeros extends southwards from the northern polar icecap. The Wall, a colossal fortification which stretches for 300 miles, separates the Land Beyond the Wall from the Seven Kingdoms, a political entity that unifies the rest of the continent.
Separated from Westeros by the Narrow Sea, Essos extends eastwards for many thousands of miles. It is larger than Westeros but less densely populated. The western edge of the continent is controlled by the Free Cities, while the city-states of Slaver's Bay are located in the south-central region. Much of the continental interior, known as the Dothraki Sea, is grassland ruled by the tribal warriors known as the Dothraki. Further east is Qarth and the Jade Sea. The mapped lands of Essos are divided by the Bone Mountains and the Krazaaj Zasqa, which run from the Shivering Sea in the north to the Straits of Qarth in the south. East of the Bone Mountains are Yi Ti, legendary Asshai, and the Shadow Lands.
Sothoros is located south of Essos, on the far side of the Summer Sea. It is a large continent consisting of deserts and jungles. It is said to be a haven for plagues and dangerous animals, and has not been explored much beyond the northern coastal regions.
Lastly, Ulthos is a large landmass located to the southeast of Essos, across the Saffron Straits from the Shadow Lands.