The term symbiote (or symbiont) refers to a Goa'uld parasite living in a host animal such as a human or Unas. In biology a symbiote is an organism that lives with or off another organism receiving and/or giving benefits. Parasitism, a form of symbiosis in which one organism benefits while the other is harmed, describes the Goa'uld relationship with their hosts because they take control of another being's body against their will.
The Goa'uld are biotrophic which means they rely on their host to survive, as the long life and physical healing benefit is provided to the host for the sole purpose of serving the Goa'uld. The Tok'ra, on the other hand, live in a mutualistic symbiosis with their hosts, because both organisms benefit by sharing control of the body. Both are called endosymbionts, which means that they live inside their hosts.
As long as a blended Goa'uld symbiote has access to a sarcophagus, its lifespan is effectively endless; however, as was the case with Cronus, Yu, and (potentially) Ra, the long-term effects of the sarcophagus eventually cause the Goa'uld to suffer a lack of reliable awareness. The Tok'ra do not use the sarcophagus for philosophical and practical reasons but were still able to live thousands of years, as long as a fresh host could be found every few centuries. (SG1: "Seth") Eventually, Tok'ra symbiotes will succumb to age.
Symbiotes are asexual and spawned from Goa'uld queens, though queens will usually try to obtain the "code of life" (DNA) from the intended host species for their offspring, so that they can more easily infest future hosts. One way for the queens to acquire the code of life is through sexual contact with a male member of that species. (SG1: "Hathor") The offspring themselves are biologically asexual. Most adopt male or female personalities and choose their hosts accordingly; some alternated for practical or other reasons.
The symbiote is one of the Goa'uld's main sources of power, because their Jaffa warriors depend on the symbiotes to survive, thus causing the Jaffa to be loyal (most of the time) to their Goa'uld masters. To the Jaffa this dependence also makes them extremely vulnerable to their Goa'uld masters, as they can be executed in two ways. They can have their symbiote removed, causing the Jaffa to die a long lingering death. Their symbiote can also be crushed within the pouch which allows the symbiote blood to mix directly with the Jaffa's blood causing an extremely painful but much quicker death. During the Summit of the System Lords, live symbiotes are taken out of a vat and eaten by each System Lord after every night of the summit. (SG1: "Last Stand")