The hostss, as they are called, are a humanoid race with either characteristic black or brown spots running down the sides of their neck or an odd forehead. The forehead type were only seen in one episode of Star Trek: The Next Generation. The spotted Trills appeared throughout Deep Space Nine.
The symbionts are helpless, worm-shaped lifeforms who contain the memories of their previous hosts, and who inhabit the abdomens of the humanoid hosts. When a host and a symbiont are joined, the resulting individual is considered a new being, and being joined is an honor that only a small portion of the race is capable of. When a host dies, the symbiont is transplanted into a new host. A symbiont who is neither implanted into a new host nor returned to their habitat (pools of nutrient-rich liquid on the Trill homeworld) will quickly die, as will a joined Trill host from whom the symbiont is removed.
In rare cases, Trill symbionts can be joined with non-Trill humanoids, but the results are unstable. ("The Host" (TNG))
A joined Trill replaces his or her family name with the name of the symbiont; for example, when Ezri Tigan was joined with the symbiont Dax, her name became Ezri Dax.
Trill society has a taboo against resuming relationships with loved ones from one's past joining, on the principle that each life must be unique. ("Rejoined" (DS9))