Trills are one of the very few joined species known in the alpha quadrant. Two separate intelligent species have evolved on the Trill home world - the hosts and the symbionts. 
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The hosts have two distinct races. The first, or alpha race, comprises approximately 10% of the Trill population; these are distinguishable by their pronounced forehead ridges. 
The second, or beta race, comprise the remaining 90% of the population. Betas are easily identifiable by a pattern of skin markings which extend from the forehead down the side of the face  and all the way down the body.  These spots move and change gradually over time, although they remain in the same region of the body.  Both of these races are known as the Trill Hosts. 
The Symbionts are small, sluglike creatures who live underwater for most of their life. The Symbionts are extremely long lived and very intelligent, but lack any kind of limbs or tool-using ability. 
Some thousands of years ago the Trill began living as a joined species - the Hosts developed the ability to physically join with the Symbionts, linking their nervous systems together.  The effects of this are different between the two races of Host; with the alphas the Symbiont totally submerges the Hosts personality upon joining, essentially using the body as a home for its own mind. Alphas tend to be less physically robust than betas; for example, many alphas are extremely sensitive to the energy processes involved in transporters, and using this technology would cause fatal damage to the Symbiont.  For this reason betas are the preferred recipients in the joinings.
Betas also have some difficulties due to the joining process. Betas become physically dependant on the Host within 93 hours of the joining; after this period, removing the Symbiont is fatal to the Host.  After a joining the mind of a beta is not submerged by the Host; the two separate minds merge, forming a new personality who is a combination of both the Host and Symbiont.  This can cause severe psychological problems if the two minds are incompatible, resulting in rejection of the Host after only a few days - a rejection which often causes the death of the Host. Only one in every few thousand Hosts are suitable to be joined, and these must go through an exhaustive series of tests and training in order to qualify. 
After the death of the Hosts the Symbiont is removed and placed into a new Host, carrying into the new joining all the personality and memories of the old. Many Symbionts have lived in this manner for centuries, accumulating experiences from many different lifetimes.