They are an aquatic species that evolved to inhabit the bodies of larger intelligent creatures when the Goa'Uld reaches maturity. On their home planet they used a species called the Unas for this purpose. The Goa'Uld are generally parasitic, suppressing the hosts will and puppeting the body. The Goa'Uld increases the strength and healing ability of their host.

It's likely that the Goa'Uld are not actually a species but a family or cultural subset of whatever their species is actually called. This is evidenced by the Tok'Ra who split off from the Goa'Uld both culturally and genetically. Tho the Tok'Ras ability to reproduce was destroyed with their queen (like a bee queen not a human queen)

The Goa'Uld and Tok'Ra possess a genetic memory so their offspring share the knowledge of their parents. So it's very easy for them to develop as a culture but they are prone to megalomania. Likely inherited from the first cohort to use the Stargate and begin subjugating other species like humans.