The inhabitants of Sothoryos are ethnically quite distinct from the peoples of Westeros and Essos. In contrast to the light-skinned and blonde-haired Andals of Westeros or the Valyrians of Essos, the inhabitants of Sothoryos are notably dark-skinned - more so than the copper/light-brown appearance of the Rhoynar, Dothraki, or Ghiscari of Slaver's Bay.
Numerous islands dot the northern coast of Sothoryos. These have been mapped, and the main island chain is known as the Basilisk Isles. West of the Basilisk Isles is the large island of Naath, whose musically-inclined inhabitants are famed for their devotion to pacifism. These islands are frequently raided for captives by ships from Slaver's Bay, which is located across the Summer Sea to the north.
The Summer Islands are known to lie off the northwestern coast of Sothoryos, far west of Naath.
When an assassination attempt is made on Daenerys Targaryen in Vaes Dothrak, she says she thought King Robert Baratheon would leave her alone after her brother Viserys Targaryen died. Ser Jorah Mormont then explains to her that as the last Targaryen, King Robert's agents will never stop hunting her no matter how far she flees, even if she goes to Asshai or the Basilisk Isles.
The far northern coast of Sothoryos appears in the title sequence once it changes to start including Qarth. Xaro Xhoan Daxos says he is originally from the Summer Islands, but rose to become a wealthy merchant leader in Qarth.
In the books
In the A Song of Ice and Fire novels, Sothoryos is loosely analogous to Africa, as Essos is to Eurasia. According to George R.R. Martin, Sothoryos is unexplored and largely unknown to the peoples living in the Seven Kingdoms and the Free Cities. What few parts they have visited on the northern coast are filled with plague-ridden jungles. The people of Sothoryos tend to be dark-skinned and many, such as the vegetarian and musically inclined inhabitants of Naath, are quite peaceful (the Naathi are vegetarians because they refuse to harm any living thing, even animals). Unfortunately this has made them prime targets for the slaving operations of the cities in nearby Slaver's Bay. As a result, it is not unusual to see dark-skinned peoples in Essos, either as slaves in Slaver's Bay, or those who won their freedom and went on to become merchants, pirates, mercenaries, etc.
The northern coast of Sothoryos lies a few hundred miles south of Slaver's Bay and Valyria. This coastline and its cities was devastated in the Doom of Valyria four hundred years ago, which triggered tsunamis that inundated the coastlands. The ruins of the once-great cities of Zamettar and Yeen can still be seen near the coast. The two cities were built along a great river (whose name has not yet been revealed): Zamettar is (or was) located at the mouth of the river along the coast, while Yeen is located much further inland, at the point where the river began to branch out into a large delta. A third major city, Gogossos, was located a little further west, on a large island just off of the main coast, known (in the present day, at least) as the Island of Tears.
The ancient Ghiscari Empire had been conducting slaving raids against Sothoryos since over five thousand years ago, much as the modern slaver-cities do. The empire even built a penal colony known as Gorosh at Wyvern Point, the northeast corner of Sothoryos near Lesser Moraq, on the opposite side of northern Sothoryos from Zamettar and Yeen. However, Gorosh was also ruined in the Doom.
The explored parts of northern Sothoryos consist of a mostly tropical climate and jungles, home to multiple tropical plagues for which outsiders have no built-up immunity. Much larger areas may have been settled before its major cities were devastated in the Doom of Valyria, but afterwards the ruins of these lost civilizations were overgrown by nature. Multiple tropical species live in Sothoryos which are not found in Westeros or Essos, including species of hairy apes, and carnivorous, roughly dog-sized carnivorous reptiles known as "basilisks" (which may be distantly related to Lizard-lions).
On the title sequence map the north coast of Sothoryos is apparently wholly located in the Summer Sea. However, when George R.R. Martin altered his maps for The Lands of Ice and Fire (a companion map collection for the novels, not the TV series) he adjusted this so Sothoryos is now located at the southwestern end of the Jade Sea. The large island of Great Moraq lies in the western opening of the Jade Sea, between Essos and Sothoryos. The north coast of Great Moraq is divided from Essos by the Straits of Qarth, while the south coast of Great Moraq is divided from Sothoryos by the Cinnamon Straits.
The names "Sothoryos" and "Sothoros" have been used somewhat inconsistently in the books. A problem is that the continent as a whole is rarely referred to by name, because almost none of the narrative takes place there. Characters from regions of the continent most often simply refer to specifically what region they are from. In the fourth novel, for example, the name "Sothoryos" is only used in-text four or five times. The few text references have alternated between "Sothoryos" and "Sothoros" (leaving out the "y"). Meanwhile, all maps have consistently used the spelling "Sothoryos" (in A Feast for Crows, A Dance with Dragons, and The Lands of Ice and Fire), though past maps have been known to include spelling errors. Moreover, on two separate occasions George R.R. Martin has answered Q&A about the continent (in 2002 and 2012), and in these cases Martin himself used the spelling "Sothoryos".
Elio Garcia (aka "Ran"), the founder of Westeros.org and co-author of The Worlds of Ice and Fire book with George R.R. Martin, weighed in on the issue: "Sothoryos" is indeed the correct spelling, and as of 2012 this is the spelling that Martin uses in all of his correspondence and maps. "Sothoros" only appeared a few times because either Martin wavered on the spelling earlier in the series, or it was an error in the editing process, but Martin has officially settled on "Sothoryos" as the proper spelling.