The Trill are a humanoid species native to the planet Trill. A small percentage of the Trill population co-exists with a sentient symbiotic organism known as a symbiont inside their bodies. The resulting joined Trill have personalities which are a synthesis of the two beings including the memories, and to some extent the personalities, of the previous hosts of the symbiont. This way, the joined being gains all the skills and occupations of the previous hosts. As late as 2367, the fact that some Trill exist as a joined symbiotic species was widely unknown, even to Federation scientists. (TNG: "The Host")
Most Trill are distinguished by two rows of spots going down each side of their body, from forehead to toe. Their skin color could vary. One of the main neurotransmitters in the Trill brain is isoboramine. (DS9: "Equilibrium") Joined Trill are also extremely allergic to insect bites; the biochemical connections between the host and symbiont cannot tolerate the reaction caused by the insect's venom. (DS9: "The Siege") One Trill peculiarity is that they are known for having cold hands. (DS9: "A Man Alone")
The few Trill that are given a symbiont are typically joined in their early to mid-twenties. The physical process of being joined is irreversible. Once joined, the host and symbiont are dependent on each other after 93 hours. If the symbiont is removed from the host, symbiont and host will die within hours, even if they are otherwise healthy, unless they are reimplanted with another host or symbiont. (DS9: "Dax", "Invasive Procedures")
The brain of a joined Trill has two cerebral nuclei and two different brain wave patterns. Julian Bashir compared them with two linked computers, which both work for the same task, in 2369. (DS9: "Dax")
Despite the relatively unusual nature of being a joined species, that aspect of their culture was widely unknown until 2367, when the Odan symbiont had to be given a new host during the middle of tense negotiations on Peliar Zel. Such a public example of the dual nature of the Trill brought to the forefront what had previously been a very private matter to Trill for several millennia. (TNG: "The Host")
In contrast to the greater revelation of their nature, the Trill are not a secretive species. To them, the joined nature of their culture is normal and not something they would think to comment on without prompting. Indeed, joined Trill can be seen as particularly genial and many have served the Federation as distinguished ambassadors, including Odan and Dax.
Jadzia Dax joked that Trill don't look for romance the way Humans do. They consider it quite a nuisance and view it as a weakness of the young. Although a Trill host may have romantic feelings on occasion, it is the symbiont's wish to live on a higher plane and to try to rise above those sorts of temptations. (DS9: "A Man Alone")
Trill law forbids reassociation between subsequent hosts of joined persons, whose symbionts were romantically involved in their previous hosts, as the main purpose of the transfer of symbionts is to experience new things in life. Trill who are found guilty of reassociation are expelled from Trill society, meaning that their symbionts die with their current host. (DS9: "Rejoined")
On average, only three hundred symbionts are available for hosting each year and about a thousand Trill apply for joining. Because there are many more humanoid Trill than symbionts, prospective hosts are weeded out by a demanding selection procedure, overseen by the Symbiosis Commission. (DS9: "Equilibrium") The competition for the few symbionts is fierce and attracts the brightest and most highly motivated of Trill society. Often the would-be hosts excel in their chosen fields and it is not uncommon for them to hold several degrees or distinctions prior to their joining. Prospective hosts may eventually become initiates under the supervision of a field docent, a joined Trill who evaluates the prospective host's suitability for joining and makes a recommendation to the Commission. A negative recommendation usually means the initiate is cut from the joining program. (DS9: "Playing God")
Common belief in Trill society holds that only one in a thousand Trill make acceptable hosts. In fact, this figure is vastly understated, and nearly half of the Trill population is capable of being joined. The myth is perpetuated very carefully, though, in order to avoid the widespread chaos which would arise if the information were made public, since the symbionts would become, essentially, objects to be fought over, as people fought to gain the few prized symbionts. (DS9: "Equilibrium")
Unless the joined Trill objects, a prospective host may request a specific symbiont. If a host is weak, the personality of the symbiont will overwhelm it. The joined Trill has several tools at their disposal for dealing with various aspects of their previous hosts. The telepathic ceremony of zhian'tara allows a current host to divest the symbiont of the personality and all the memories of a previous host, which are temporarily hosted in volunteers. The transferring process is performed and supervised by a symbiont Guardian, an unjoined Trill telepath. The ceremony creates a chance for closure by having the new host address the previous hosts directly as a means of distinguishing their voices and cementing the sense of finality of its latest transition. Similarly, the Rite of Emergence can focus the voice of a single previous host among the memories of the other, allowing for a more direct converse in times of need for the current host.
The zhian'tara ritual is roughly similar to the Vulcan fal-tor-pan ritual, since both enable the respective species to perform a synaptic pattern displacement, the transfer of what could be considered as a soul (katra, pagh, etc.). (DS9: "Invasive Procedures", "Facets", "Field of Fire")
In the mirror universe, it was unclear which side (if any) the Trill government took in the conflict between the Klingon-Cardassian Alliance and the Terran Rebellion. A number of individual Trill were part of the Rebellion, while others would attempt to remain non-aligned. (DS9: "Through the Looking Glass")
Ships and technology
It is not known when Trill and Humans first made contact, but Tobin Dax once met the famous Cardassian poet Iloja of Prim while the latter was exiled on Vulcan, indicating Trill and Vulcan contact prior to 2245. (DS9: "Destiny") Emony Dax is known to have visited Earth in the mid 23rd century, where she was apparently close to Leonard McCoy. Travis Mayweather mentions having visited Trillius Prime, but it is not made explicitly clear if this is the same planet as the Trill homeworld.
The Trill hosts seen in "The Host" featured a different make-up style than the more common spots seen on Deep Space Nine. The reasons were purely cosmetic and the writers of DS9 were aware of the contradiction, but no canon explanation has ever been offered. After she had put on the Odan forehead appliance, someone looked at Terry Farrell and said to Michael Westmore, "What did you do to her head, she used to be beautiful?" Westmore then suggested to "just give her spots like we gave Famke," who played a Kriosian. This makeup was used on all Trill afterwards. (Star Trek: Deep Space Nine Companion)
However, the disparity between Trill seen on TNG and the later Trill goes beyond makeup. "The Host" seemed to make it clear that the symbiont was dominant (in fact, the symbiont is referred to as a parasite on more than one occasion), and that the host was merely a body with no influence over the joined entity. Odan was apparently unable to safely use the transporter while Dax used it often with no trouble at all. Odan was ready to pick up where he/she and Crusher had left off despite the Trill taboo against reassociation.
A link between the Trill symbionts and the alien parasites seen in TNG: "Conspiracy" is suggested in the non-canon DS9 short story "Sins of the Mother" (in the anthology The Lives of Dax) and the novel Unity. This was expanded upon in Worlds of Star Trek: Deep Space Nine, Volume 2. The neural parasite was first seen as it traveled in a comet passing close to the Trill homeworld. Audrid Dax and her husband, Jayvin Vod, went to the comet with a Federation team to examine it when Trill scanners showed a being similar to a symbiont in the comet. However, as the Federation was unaware of the Trill's symbiotic relationships yet, it was kept in secrecy. Upon coming into contact with the chamber of the parasite, it bonded with Jayvin, as well as the Vod symbiont, ultimately causing both of their deaths.
The inconsistencies between TNG and DS9 Trill were dealt with in the novels by suggesting that ridged Trill and Trill who could not be transported were subgroups of Trill society; both were mentioned or shown during the scenes on Trill, and the inability of some Trill to be transported was a plot point. In the DS9 Relaunch novels dealing with Trill, the ability or inability to be transported is shown as simply a difference between various Trill ethnicities.
The comics took a far different course, such as the Star Trek: Divided We Fall mini-series, where the fact is completely ignored. In fact, both Odan hosts are shown, without facial ridges but with spots. All Trill in the series are shown with the spots instead, no reasoning given.
In another novel, Forged in Fire, the ridged Trill are said to result from a strain of the Augment virus that managed to infect a Trill colony through visiting Klingon traders. However, the Trill seemed not to be interested in working on methods of restoring the original Trill look, with this "sub-group" having recently (at the time the novel was set) being re-accepted back into Trill society. This division among the groups possibly explains why Odan was so different from other Trill encountered in the series.