On their homeworld, the Goa'uld are born as relatively helpless aquatic larvae, spawned in large numbers by a queen. Those which survive to maturity develop into muscular serpentine parasite with fins and fangs that allow them to spring powerfully out of the water to burrow into a suitable more mobile host as it requires a host body to inhabit in order to affect the environment around it. The symbiont typically enters the body of a host either through the mouth or burrowing through the neck. Once inside the body, they attach themselves to the spinal column and begin their control over the body. Only 50% of the Goa’uld then succeed in taking over their host.
One of the unique traits among the Goa'uld is their genetic memory which allows them access to the accumulated knowledge of their race from the Queen that spawned them.
The primitive reptilian humanoid Unas that lived on the same world were their original hosts. Through the abandoned Stargate that was built on their planet by an alien much advanced civilisation, they emigrated to other worlds, infiltrated and invaded other alien species and used their technology. Eventually, around 8000 BC, they discovered Earth. Humans proved to be a most suitable host-race for them, as human bodies were easy to repair, and human hands and voices offered great opportunity for expression and technological use. Once a Goa'uld takes a host it is not easy for it to switch to another; it loses its fins and its body appears to atrophy significantly. So civilised Goa'ulds developed an alternative lifecycle; modifying a race of humans into what they called Jaffa to act as incubators for their larvae. This provided them powerful warrior servants and improved to almost 100% the chances of the larvae taking over a human host on maturity. The Goa'uld were never a numerous species, never more than some thousands of adults at any time.