The Trill were a humanoid species native to the planet Trill and members of the United Federation of Planets. The Trill had a unique relationship with the sentient Trill symbionts of their world, which saw them able to act as hosts for the symbionts. A Trill host when joined with a symbiont would gain the memories of the symbiont allowing them to gain the memories and skills of the symbiont's previous hosts. As such it was a highly sought after and valued experience amongst the Trill.
Trills were distinguishable by the pattern of spots that run from the side of their forehead and down their body to their feet. (Star Trek: Deep Space 9) A small percentage of the Trill population lacked these spots and instead had a forehead ridge. (TNG: "The Host")
The joining between a Trill and symbiont involved inserting the symbiont into a stomach pouch of the Trill. The symbiont would then form a connection with its host allowing the host Trill access to the memories of the symbiont. After being joined for 93 hours it became permanent for the Trill as they became dependent on the symbiont and its removal would quickly lead to the death of the Trill if it was not returned or a new symbiont found.
The Trill believed only a small percentage was suitable for joining when in reality around half the population was. However, due to the value of the symbionts and there much more limited number this fact was kept secret from the Trill population as a whole. (DS9: "Equilibrium")
Prior to 2367, knowledge of the Trill and their relationship with the symbionts was not widely known to non-Trills. Even Starfleet was largely unaware of this despite the presence of Trills in Starfleet and serving the Federation in various roles such as ambassadors. (TNG: "The Host")