Hydrans are short beings (about four feet tall), greenish-gray in color, with a barrel-shaped body supported by three stubby legs. Three equally-spaced tentacles sprout from the top of the body. Hydrans are neckless. Above the tentacles, two mouths, two nostrils, two eyes, and two ears are grouped in a fleshy mound on top of the body.

They breathe methane (which provides most of their food). Their home world is a type-S gas giant, somewhat smaller than Uranus, with rocky continents floating on a sea of liquid methane.

Hydrans have three sexes, which are generally referred to as "male", "female", and "matriarchal". Males tend to dominate the command and technical positions, while females form most of the labor force, fighter pilots, and ground combat troops, but some do hold command positions or even the throne. Matriarchals have the intelligence of a sheepdog and never hold positions in business or industry; some sterilized matriarchals hold servant positions on starships.

The Hydran government is a monarchy. The lines of succession are quite complex since the Hydrans are anything but monogamous. There is an extensive nobility that involves itself in internal politics and a strong civil service that actually runs the day-to-day affairs of government. The numerous trade guilds hold considerable economic power and often control who ascends to the throne.

The Hydrans are constantly at war with their neighbors, the Klingons and Lyrans, and were conquered by those empires twice during recorded history, but managed to overthrow the invaders from secret bases in the Lost Colonies, worlds that the invaders never found.

The early Hydran ships (from the first war of liberation) were armed with short-ranged fusion beams and Gatling phasers. Most Hydran ships carry fighters which are used to supplement the firepower of the warship itself. As these fighters carry fusion beams (and, after Y170, Gatling phasers), they are deadly and a small squadron firing from short range can cripple any cruiser in the galaxy. The tactics of these Hydran ships revolve around bringing the fighters into action and preventing them from being destroyed before they can fire. Later ships carried the longer-ranged hellbore cannon and fewer (or no) fighters.