The Thoom are semi-amphibious humanoids, averaging around 5 feet tall as adults. Thoom eggs hatch in nursery ponds, and baby Thoomlets can breathe in either air or water for the first five years of life. Around age five, their gills transform into throatsacs, and the ability to breathe underwater is lost. From age five through adulthood, they can still stay underwater for extended periods of time, however, by inflating their throatsacs with air before submerging. The "thoom" noise made during throatsac inflation is what provides the race's name. This noise, called "thooming," is also a form of communication.

Thoom skin color ranges from pink to brown to green. Blue Thooms are rare but not unknown. Thooms prefer to keep their skin damp and are often found near lakes, rivers or the sea.

Thoom eyes are very large and are widely spaced on the sides of their heads. This provides them with an excellent field of vision but poor depth perception. As a result, Thooms must work harder than other races to become effective warriors.

Female Thooms look very similar to males, except they tend to be somewhat shorter and less broad-shouldered. They have narrow waists and a crest on top of their head.

The Thoom are highly intelligent and well-travelled, yet somewhat melancholy and bashful by nature. They are widely known as the world's leading scholars and sailors. Thoom universities are unrivalled, and students from distant lands visit them as part of a Thoom student exchange program.Thoom medicine is particularly advanced, and Thoom healers are always in demand.

The Thoom prefer to avoid political entanglements with other races. Their extensive knowledge of history and psychology gives them a sharply world-weary sense of having seen everything before, and as a result they have a very jaded, cynical perspective on world affairs. (The loud Thoom sigh and eye-rolling expression are well-known by the other races.) They tired long ago of fighting wars and having their libraries and sailing ships burned and ravaged (and tired of destroying those of others as well) and so they now simply allow whatever political entities in ascendance at the time to bloodlessly "conquer" them. The Thoom are careful to orchestrate such "conquests" in such a way that they actually maintain their autonomy, and proceed to maintain their largely benevolent mastery of the world's economic and intellectual affairs. In general, the other races tend not to provoke the Thoom, for two reasons. One reason is that the Thoom presence behind the scenes in economic, scholarly, and cultural affairs is recognized as essential. The other reason is that when provoked, the response of Thooms is typically extraordinary and devastating. After the Thoom-Sylvan conflicts of 351 BA, Thoomish zoologists unleashed a massive army of hungry beavers on the Sylvan forestlands. The Sylvans sued for peace almost immediately.

Domestic Thoom politics are best characterized as primarily family-oriented, with the larger and more well-known families heavily represented in a Council of Elders. Smaller families are also guaranteed representation as well. Male and female Thooms are equally represented in the Council. Due to Thoom political cynicism, many Thooms ignore their own government entirely, and this is considered acceptable if done conscientiously.

Though at times vain and occasionally condescending, the Thoom do in fact concern themselves deeply about the welfare of the other races. Thoom doctors have made significant strides in medicine and health care in the past century, and travelling Thoom healers have worked hard to spread these advances throughout the world.