A Yeerk is a gray-green sluglike creature, perhaps a few inches long at most. They have sensory stalks like slugs, but, unlike slugs, have no eyes. Also unlike slugs, they have no mouths, have two "flippers" on each side, and are covered with small, purple "osmosis nodes."
Yeerks are photosynthetic, like plants. However, they specifically require the rays of their home sun, called Kandrona rays, and require something in the so-called "Yeerk pools" to synthesize them. The osmosis nodes, therefore, serve the same function as roots do in plants. Yeerks solve the problem of their food's rarity by creating artificial Kandrona rays.
Yeerks can infest an individual by entering the ear canal and flattening itself so as to fit in the folds. From here it is able to sieze total control of all conscious and some subconscious functions, such as moving, speaking, breathing (at least to unconsciousness), and dreaming, the last of which they often used to "break" their hosts. They cannot control unconscious functions (e.g, a Yeerk cannot cause its host's heart to stop unless the host was consciously able to do so before infestation). Yeerk hosts are called Controllers. A notable weakness of infestation is that Yeerks cannot inhabit hosts indefinitely, as they must feed every three days or die. If the host eats oatmeal regularly, the Yeerk cannot control the host, but remains alive until the host stops eating oatmeal (whereupon the Yeerk reasserts control and the three-day feeding cycle resumes).
Yeerks reproduce in threes, as opposed to twos as with humans and most other organisms. Any three Yeerks can fuse together into a single parent organism, which then splits into a brood of approximately six hundred.
The Yeerk homeworld is, as far as is known, a foggy, barren world dotted with Yeerk pools and bathed in Kandrona rays. It is also inhabited by a (barely) sapient race called Gedds, who serve as the main hosts for low-ranking Yeerks.
Yeerks are led by the red-robed Council of Thirteen, which is led by the Yeerk Emperor. The Emperor's identity is known only by its fellow councillors. The army is led by warlords called Vissers, ranked from Visser One downwards. Below the Vissers are the Sub-Vissers, ranked in the same way. Yeerks use their pools as a combination church/mess hall, where they feed and worship their god Crayak. Despite this, they are not a theocracy. They are imperialist and have conquered several races, including the ravenous Taxxons and the blade-covered Hork-Bajir.