Trill

The Trill are a humanoid species native to the planet Trill. Some members of the species co-exist in a symbiotic relationship with a sentient organism known as a symbiont.

Society
While it wasn't widely known until 2367 about the Joining Process, Trill had never been a very secretive society, though they had long considered their dual nature to be private. In fact, many Trill both Joined and Unjoined have served the Federation as distnguished ambassadors in the past such as the Dax and Odan symbionts. (DS9, TNG)

The exact time period that Trill made contact with Earth and later the Federation is up for debate, but the Star Trek: Destiny novels suggest it might have been as early as the 2150's. (Star Trek: Destiny)

Among Joined Trill, reassociation or basically falling in love with someone a previous host loved is forbidden, and is punished by exile, which typically results in the death of the symbiont. (DS9: Rejoined)

Joining
On average, only about 300 symbionts are available for the joining process each year. Prospective hosts are chosen by the Symbiosis Comission, and are usually put through rigerous testing to decide if they are capable of being Joined. Common belief among the Trill populace is that only one in 1000 Trill are capable of being Joined, but the truth is that the number is closer to half the population (DS9: Equilibrium).

Unless the joined Trill objects, a prospective host can request a specific symbiont. Weaker hosts tend to be overwhelmed by the symbiont's personality, and hosts have different ways of coping with the memories of the previous hosts.

One of the most important parts of the process is the Zhin'tara ritual, where an unjoined Trill makes a connection between the current host and a friend or relative, temporarily placing the personality and memories of a previous host into the other. This is a way for hosts to gain closure, and is required by the Symbiosis Comission.

Trill in Starfleet
The First Trill Starfleet officer was Ensign Lera Kesta who served as the Communications officer on board the NX-04 Discovery. The first Trill Starfleet Captain was Naima Rosa on board the USS Republic, both where later revealed to have been joined. (Star Trek: Discovery)

Two hosts of the Dax symbiont, Jadzia and Ezri Dax both served on Deep Space Nine before and during the Dominion War. (DS9)