Trill was an M class planet, 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: "Equilibrium")
In 2375, the USS Destiny was en route to Trill with the Dax symbiont, which had been injured, when the entity took a turn for the worse. As a result, it had to be joined with Ezri Tigan. (DS9: "Shadows and Symbols")
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In the series bible for Star Trek: Deep Space Nine, it was said that the underground of Trill and the planet's surface were, "many centuries ago," 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 more 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)
According to the Star Trek: Star Charts (pp. 36 & 44), Trill, or Trillius Prime, was governed by the Trill Symbiosis from their capital city of Mak'ala. There were 650 million Trill and 11 million Symbiont living on this world in 2370. This civilization had warp capability since antiquity, and were admitted to the Federation in 2285. Trill was a stop on the Earth trade routes in the 22nd century.