
Physiology
Trill physiology is somewhat unique from standard Terran. To begin with, there are two distinct forms of Trill humanoid. First there's the typical 'spotted' Trill, but there's also ridged Trill.

Trill spots run from the forehead all the way down. Ridged Trill
too, have spots below the neckline.
A trill's most distinct features pertain to the symbiotic nature.
The trill people have a small abdominal pouch, which can be used to host
a symbiot. Because of their symbiotic nature, some joined trill have
adverse effects from using transporters, up until advances in the technology
around 2340, yet some host still refuse to use them. Joined
trill are also highly allergic to insect bites & have noticeably cold
hands and feet.
Customs
trill had been an isolationist world until around 2100, earth time. The people feared for the safety & sanctity of the symbiots if outworlders were to be let planetside. This was around the same time they began allowing women into their ruling council.
Trill people can sometimes be taken (sometimes deservingly so) as cold or arrogant to non trill. Their personalities tend to be rather reserved & quite accomplished & well studied.
Around the age of twelve, most trill go through a mandatory genetic testing, to establish their suitability for joining. Although the testing isn't mandated, almost the entire planet's population undergoes these tests.
Reconnecting with a former hosts loved ones is considered strictly taboo.
Trill names are given then family, as with terrans. when one becomes joined, the family name is replaced with that of the symbiot.
Joined trill rarely engage in romantic endeavors. they tend to consider it the province of the young. Most, but not all, consider it folly or weakness & prefer to spend time with 'higher pursuits'. Occasionally, a host will indulge in an affair, or affairs, anyway.
legislature
Trill is ruled by a governing council of citizens from around the planet. This group is made up of joined & unjoined trill. There are some cases of prejudice against unjoined or first host trills. Unjoined & newly joined trill have an uphill battle before them with some members of the government, as many consider them to be of lesser intellect or ability. These Trill have to work twice as hard to prove themselves.
the symbiosis commission
the symbiosis commission is a group of doctors & other experts who screen potential trill candidates for joining. their primary goal is the care & proper placement of the symbiot. Publicly, the commission is a fairly benign organization that claims that only 10% of the people could suitably support a symbiot. secretly however, the number is closer to 50% of the population.
the discrepancy comes from the commission's high regard for the symbiots. only a few hundred symbiots are available to be joined, to the millions of trill people. The commission fears that if the truth were revealed, the symbiots would be in danger of becoming commodities, bought, sold, mishandled & fought over, rather than seen as a revered & greatly deserved honor.
The other function of the commission is to administer the host initiate program. this involves the testing & screening of thousands of hopefuls a year. once selected for the program an initiate, usually in their mid to late teens, begins training & study. After a time they are assigned a field docent. it's the docent's job to evaluate their progress & prepare them for joining. At any point an initiate can be cut from the program. it is almost unheard of for an initiate to reapply & be accepted as a host on the second time.
the guardians
a symbiot pool
the guardians are a group of unjoined trill that dedicate their lives to the care & well being of the symbiots. they work in the Mak'ala caves where the unjoined symbiots are kept. the guardians monitor & regulate the conditions of the pools the symbiots live in until joining.
it's said that a guardian can know a symbiot better than it's host.
they have their own connection & communication with the symbiots &
can often tell which symbiot a host is joined to, just by looking.
the joining
once
an available symbiot is ready to be joined, a candied is selected from
thousands of initiates. ideally, the initiate are both together &
prepared for the change, but this isn't always the case. once joined,
the pair become interdependent after 93 hours & separation will mean
death for the host, if not host & symbiot. the symbiot
can be placed in another species temporarily, but will only live a matter
of days in stasis.
upon joining, the change in the host is almost immediate. the host & symbiot blend together, forming a new person, different in many aspects than either were before. Upon joining, trill are considered a new person, neither legally or morally responsible for the previous obligations of either symbiot or host. A new host may retain a large portion of their original personality, in fact needing to be strong willed so as not to be overwhelmed by joining, but can pick up quirks or interests of previous hosts.
The symbiot retains the memories & experiences of previous hosts. During a scan, joined trill come up with two separate brain wave patterns, once for the host & one for the symbiot.
a
trill symbiotThe symbiot itself is a small, motile & blind. It can survive only hours outside a host or the pools on the trill homeworld. while in the pools, symbiots communicate with each other through small electrical charges. even after being in a host, or even several host, a symbiot may be returned to the cave pools to recover from serious trauma.
geography
With such little information out about trill, there's not much known about their world. Trill is just 37 hours from DS9. It's an M-Class world with purple oceans. Some of the places of note are the capitol city, home of the government & symbiosis commission, the mak'ala caves were the interconnecting subterranean symbiot pools are & the ice cliffs.