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. Both are called endosymbionts, which means that they live inside their hosts.
Symbiotes are asexual and spawned from Goa'uld queens. However, they usually have male or female personalities and choose their hosts accordingly.
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. 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")