A symbiont was a sentient vermiform lifeform from the planet Trill. It was transferred from one humanoid Trill host to another, through a surgical procedure, upon the death of the prior host. The symbiont took with it all of the memories of the previous Trill hosts so, upon joining, the new Trill host gained those memories.
The symbionts lived in the Caves of Mak'ala on Trill and were cared for by Guardians. The symbionts communicated with each other via electrical impulses transmitted through milky water that they inhabited, and apparently could speak with the Guardians in the same manner. They were long-lived compared to most humanoid species, and could easily live beyond 550 years.
Once a symbiont was joined to a Trill for over ninety-three hours, the symbiont became dependent on its Trill host and vice versa; the Trill would die within hours without a symbiont, and the symbiont could not return to the Caves of Mak'ala (if joined for the first time) nor survive a significant period of time outside of a host.
The Symbiosis Commission oversaw the joining of the vermiform symbionts and humanoid Trills. In the 24th century, about 500 symbionts became available for joining, each year, compared to approximately 5,000 Trill becoming initiates per annum. The relatively low quantity of symbionts resulted in Trill society hiding from their general populace the fact that virtually 50% of the population was suitable for joining, and the general belief in the 24th century was consequently that only one in every 1,000 Trills was suitable for joining. (DS9: "Equilibrium")
Although Humans could be used as a host to a symbiont, the symbiont could not remain in their bodies for long without physically harming the Human host. (TNG: "The Host") A symbiont could survive normally in a partly Trill host, however. (DS9: "Children of Time")
Rites and ceremonies Edit
Background information Edit
For TNG: "The Host", Michael Westmore based the design of the symbiont on a caterpillar's body attached to the head of an octopus. ("Michael Westmore's Aliens: Season Two", DS9 Season 2 DVD, special feature) A few convolutions were added to make the appliance look as if the symbiont had lots of brain matter. An air bladder was inserted into the symbiont's head and pulsated to give the impression that the symbiont was alive. The head was also colorized with a fluorescent paint, making it glow when exposed to black light during a surgery scene. (Star Trek: Aliens & Artifacts, p. 117)
For the second appearance of a symbiont, in Star Trek: Deep Space Nine pilot episode "Emissary", Michael Westmore changed the design significantly. The symbiont was "still in a similar vein shape-wise," he stated, though the revised version was smaller than its predecessor. (The Official Star Trek: Deep Space Nine Magazine, Vol. 1, p. 28) "One [reason for changing the symbiont] was to give it more of a streamlined look [....] The way this [altered version] was designed, it was almost like a roundish type of a triangle, with these little feelers that would bend back," explained Westmore. "And by putting a lot of KY Jelly on it whenever we would do the effects, it would slide a lot easier." ("Michael Westmore's Aliens: Season Two", DS9 Season 2 DVD, special feature)
The series bible of Star Trek: Deep Space Nine stated, "The symbiont is an invertebrate, androgynous lifeform that lives within the host. It looks like a short, fat snake. Many centuries ago, the symbionts lived underground [...] [until joining with the hosts] due to an environmental disaster." 
In the first draft script of "Trials and Tribble-ations", a hypothetical evolutionary link was made between Trill symbionts and tribbles, as some xenobiologists had apparently theorized that symbionts were evolved from tribbles. Jadzia Dax consequently referred to the symbiont inside herself as "a very smart Tribble."