This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
Trill was an inhabited class M planet in the Alpha Quadrant. It was the homeworld of two sentient species who shared a symbiotic relationship: the Trill, a warp-capable humanoid species, and the Trill symbionts, who were non-humanoid. (DS9: "Invasive Procedures", "Equilibrium")
Points of interest
Institutions and agencies
In 2375, the USS Destiny was en route to Trill with the Dax symbiont, which had been injured, when the symbiont took a turn for the worse. As a result, it had to be joined with Ezri Tigan, the only Trill on the Destiny. (DS9: "Shadows and Symbols")
The Burn in the late-31st century decimated the Trill population, resulting in the planet becoming more insular in an attempt to starve off extinction. Despite this, a century later there were still not enough viable hosts for the remaining symbionts.
In 3189, the time-displaced USS Discovery traveled to Trill to help Adira Tal access the Tal symbiont's memories. (DIS: "Forget Me Not") After the defeat of Emerald Chain, Trill rejoined the United Federation of Planets. (DIS: "That Hope Is You, Part 2")
In the series bible for Star Trek: Deep Space Nine, it was said that the underground of Trill and the planet's surface were where the Trill symbionts and humanoids lived, respectively. It was also stated that "an environmental disaster" led the two species to begin integrating with each other. 
As there were two make-up configurations designed for the Trill, Jadzia Dax actress Terry Farrell once suggested that "the north of Trill" was the birthplace of the simpler-looking Trill whereas the planet's south was where the more elaborate physical design, involving spots, originated. (Cinefantastique, Vol. 23, No. 6, p. 21) While filming DS9 Season 1 finale "In the Hands of the Prophets", Farrell also expressed hopes of seeing Trill in the second season of Star Trek: Deep Space Nine, saying, "I'd like to go to the planet where I'm from." (Cinefantastique, Vol. 24, Nos. 3/4, p. 98.)
The name of the Trill homeworld was first established as "Trill" in the third season outing "Equilibrium", which also marks the first time the planet appeared on Star Trek: Deep Space Nine. Portraying the planet in that episode proved highly expensive. Some of the Trill exteriors shown in "Equilibrium" were filmed on location in Huntington Gardens. "I think there were a lot of interesting things about the Trill homeworld that you found out in the episode," commented René Echevarria, who wrote "Equilibrium"'s script. (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, pp. 84 & 85.)
Star Trek: Star Charts (p. 33) and the Star Trek Encyclopedia (4th ed., vol. 2, p. 421) designate the Trill homeworld (Trillius Prime) as an M-class planet governed by the Trill Symbiosis, admitted into the Federation in 2285. The planet's capital was Mak'ala. Points of interest noted included the Hoobishan Baths, the Tenarian (Tenaran) Ice Cliffs, and the Caves of Mak'ala. As of 2370, the planet was populated by 650 million Trill and 11 million symbionts, though the existence of symbionts was not widely known prior to 2367.
According to Star Trek: Star Charts (pp. 36 & 46) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the Trill system was located in the Alpha Quadrant. This was a binary system with a G-class star and a K-class star. The system was a destination on an Earth trade route in the 22nd century and on a major space lane in the late 24th century.
The Star Trek: Federation Travel Guide calls out local attractions including the Teneran Ice Cliffs (3/3 stars) in the southern hemisphere and the caves of Mak'ala (2/3 stars.) "The Interplanetary Bar and Grill" serving foods from hundreds of worlds is highlighted, noting "local cuisine failed to tempt our reviewers palate." The Selipsis is identified as Trill's largest and oldest beach resort. "Grow old in wisdom" is noted as a local key phrase to know.