Dax was a Trill symbiont, a symbiotic lifeform that existed with the humanoid Trill as a joined species. Each joining of Dax with a host created a new, unique individual, but each individual also carried the memories of the previous hosts.
As of 2375, the Dax symbiont had been joined to nine official hosts, starting with Lela Dax through its latest host, Ezri. Additionally, the Dax symbiont was temporarily joined with Verad during Jadzia's first year on Deep Space 9, and in an alternate timeline was later joined with Jadzia's descendant, Yedrin.
Dax was born on Trill in 2018. By 2374, it had been married six times – twice as a groom, and four times as a bride. (DS9: "You Are Cordially Invited") Dax had a total of nine children – five as a mother, and four as a father. (DS9: "Time's Orphan") Having been a mother three times, and a father twice, Dax claimed "I wasn't very successful either way." (DS9: "The Nagus")
Dax has held numerous positions of distinction throughout its many lives including head of the Trill Symbiosis Commission, the leader of the Trill government, Federation ambassador, and two careers in Starfleet. (DS9: "Facets")
|The hosts of Dax|
|Lela • Tobin • Emony • Audrid • Torias • Joran • Curzon • Jadzia • Ezri • Yedrin • Verad|
It is not known when the Dax symbiont was first joined to a host. However, it is unlikely that the first joining occurred as early as 2018 because this would mean that the first four hosts of the Dax symbiont served as host for an average of nearly 66 years, from 2018 until 2284 when Torias Dax became host. Given that Curzon Dax served as host for over eighty years this may be possible, but seems somewhat unlikely as several hosts of the symbiont – notably, Torias, Joran, and Jadzia – served in that capacity for less than a decade.
Moreover, the third Dax host, Emony, is known to have served as host in 2245, over two hundred years after the Dax symbiont was born. Had the symbiont been joined as early as 2018 the first two hosts, Lela and Tobin would have served for an average of nearly 110 years apiece. It is also notable that Dax had served as field docent for two hundred years as of 2370, suggesting that the first joining for Dax occurred no later than 2170. Also, it appears that newly born symbionts live for a period in the breeding pools in the Caves of Mak'ala on the Trill homeworld. (DS9: "Equilibrium")
The British comedian Bill Bailey (who once hosted the show Space Cadets, which featured shows called "Vulcans vs. Klingons" and "Quarks vs. Roms") named his son Dax after the symbiont. He commented "Yes, he's called Dax after a symbiont, a creature that absorbs the power of others. I may just have given him too much baggage. I'll tell him he's named after the German stock exchange". 
The symbiont's nature has saved its host's crewmates- and once even saved the universe- on at least two occasions in spin-off media. In the series Star Trek- Invasion!, where the first four crews face a race known as the Furies who once ruled the Alpha Quadrant, a temporal disruption of the wormhole results in the Defiant being sent back over five thousand years into the past, with the only 'survivor' into the present being the Dax symbiont, kept in stasis for centuries by the alternate Julian Bashir, allowing it to pass on vital information to its past self to stop the events that lead to the destruction of the Defiant, as well as to take control of the 'Unclean'- a race who absorbed others into its mass, who had been responsible for the Furies leaving the Alpha Quadrant- and destroy them. During Star Trek: Deep Space Nine - Millennium, when the crew were briefly trapped in Pah-wraith-inspired Hells to keep them occupied in the last dregs of real space/time existing after the destruction of the universe, Jadzia Dax was able to escape her Hell and free her friends in the same way because the Pah-Wraiths were trying to torment all seven of her past hosts and Dax itself, causing an overload of information that allowed Jadzia to work out what was happening.