The biology of the Goa'uld is complex to describe, mostly because of the intervening with the human or unas subjects. The symbiotes are born from a parent called the Goa'uld Queen. There are actually several known queens and each one capable of giving birth to thousands of symbiotes. The symbiote gets the genetic memory of the queen, who thereby imprints their way of life in a very effective way. The Queen is capable of self-fertilization, and usually will try to acquire the DNA of the species the symbiotes are intended to take as hosts, though it is not necessary, as the "wild" Goa'uld of the Goa'uld homeworld took human hosts without their queen having access to human DNA.(SG1: "Crossroads", "The First Ones") Egeria was a Goa'uld queen who turned against the majority of the Goa'uld in issues regarding the exploitation of the so-called subjects. She therefore imprinted on her young her own point of view which led to the creation of the Tok'ra. (SG1: "Crossroads", "Cure")
After they are born, it takes some time until the symbiotes can do anything useful with their knowledge, so these are often placed in the abdomens of the Jaffa slaves for several years, until they are ready to take a host. This is done to provide them with the electrical impulses that will keep them alive, and increase their chances of successfully blending with a human host. While within a Jaffa, the symbiote, called a prim'ta, will function as the Jaffa's immune system, giving them perfect health and an extended life span. It also binds the Jaffa to the Goa'uld, as their own immune system begins to break down after achieving puberty. However, it is not necessary to be incubated within a Jaffa, as the Goa'uld on the Goa'uld homeworld took human hosts without being inside a Jaffa.(SG1: "Crossroads", "The First Ones") Observe that the Tok'ra doesn't take hosts, rather they co-exist with their hosts, when both parties are willing to do this. (SG1: "Children of the Gods", "The Tok'ra, Part 1", "The Tok'ra, Part 2")
Symbiotes have regenerative abilities and with the aid of Sarcophagi they are capable of living thousands of years within the same human host. (SG1: "Serpent's Song") Even without a Sarcophagus, a symbiote's natural regenerative abilities can sustain a host for around 400 years or so, and the symbiote itself can survive for multiple millennia as long as it switches host bodies when a new one is required (Selmak's natural lifespan was roughly 2000 years, and the Tok'ra believed that Setesh could have survived on Earth for the 5000 years since the Tau'ri Rebellion even without possessing a Sarcophagus). (SG1: "Seth", "Threads") When a Goa'uld symbiote dies, it releases a toxin into the host's bloodstream, killing the host. It is possible for the symbiote to not release the toxin, but this takes a conscious effort. (SG1: "Summit", "Last Stand", "Endgame", "Threads")
While in a host, a symbiote can greatly enhance the host's strength, healing abilities, and intelligence. They are also capable of making a hosts eyes glow (how is still unknown) and usually talk in a deep, echoing voice.