The Goa'uld are a species of eusocial parasitic sapients that are the first revealed and primary villains for much of the Stargate franchise.
Goa'uld resemble lampreys and are about thirty centimeters long. They are capable of entering the body of a humanoid to commandeer control of it. The personality of the host remains intact, although the parasite has complete control of the being's motor controls. Once established, the parasite increases the host's lifespan, strength, and general health. Advanced technologies, such as sarcophagi, further increase the host's life expectancy to practical immortality while the symbiote itself can live unaided for millenia if it moves from host to host. Surgical Removal of a Goa'uld Symbiot without killing the host is possible, although it is an extremely difficult procedure. Typical (although not universal) Goa'uld personality traits include an intense hunger for power, arrogance, a short temper, antisocial behavior and a tendancy towards the over the top and grandiose.
Goa'uld reproduction is done via Queens, which are capable of producing large quantities of offspring as well as being able to pass on a genetic memory to their offspring and are capable of selectively altering them, being able to produce Goa'uld born scientifically literate as well as drones with highly limited minds and mindless blanks. Most Goa'uld Symbiotes never take a host, and are eaten by the hosts of established Goa'uld Symbiotes. Why the goa'uld engaged in such acts of cannibalism is unknown (outside of population control).
Most Goa'uld have Naquadah in their blood and release trace amounts of it into that of their hosts', although this is not vital to their survival. The Goa'uld occasionally install sensor systems geared to detect said Naquedah into certain machines such as Hand Devices for security purposes.
Goa'uld have an ability to sense a Goa'uld symbiote in close proximity. Jaffa and former Goa'uld hosts also possess this ability, although to a lesser degree. The mechanism of this detection has never been revealed.
Native to P3X-888, the Goa'uld emerged as a powerful faction after the collapse/Ascension of the Ancient civilization. Using the Stargate network, the Goa'uld managed to come across ancient Earth and posed as gods among several early Terran civilizations. They relocated populations of humans to various other worlds, transplanting human civilizations across the galaxy and eventually abandoning Earth. The Goa'uld established a feudal empire over most of the Milky Way, keeping themselves in power via armies of Jaffa and religious indoctrination, with human serfs under the Goa'uld worshiping them as gods.
Goa'uld society was feudal, but far from united. The Goa'uld sphere of influence was divided into different kingdoms consisting of several planets ruled by a System Lord, who in turn had several administrators ruling over smaller domains within the kingdom, as well as subordinates who served the system lord directly. Various staffs of officers, administrators and scientists served these administrators. Beneath them were the Jaffa, who served the Goa'uld in a military capacity as soldiers and enforcers, and human serfs and slaves. Occasionally a single powerful (such as Ra) System lord would emerge and hold considerable sway over the rest of the system lords, although this was a purely informal arangement. Administration was often rather loose, with many worlds simply being abandoned and left to their own devices after their usefullness expired.
Under normal conditions, the Goa'uld system lords were constantly at each other's throats, vying for power in a semi-ritualised form of warfare, cooperating only in the face of major threat to their status-quo. Should one system lord die, his/her domain would be carved up among the other system lords. Presumably new System Lords theoretically could rise to prominence and lay claim to their own domains, but this has never been observed and the established System Lords have been in power for millennia. To prevent potential threats from cropping up, the Goa'uld banned scientific and technological learning among their subjects and any civilization that was deemed hazardous was to be quickly dispatched.
Despite the fractured nature of goa'uld society, the Majority of the System Lords in the Goa'uld Sphere of Influence were willing to collectively make agreements in responce to external forces, ethier diplomatically (such as the Asgard) or with military force (such as with Anubis) in a very loose alliance. A few Renegade System Lords existed outside this alliance however, generally after being exiled after being thrown out after a military defeat.
At their peak, the Goa'uld collectively had a vast interstellar domain under their control and were the dominant faction in the Milky Way galaxy, ruling over billions of human serfs and thousands of star systems. However, most of these worlds were at an Iron Age period of development, although there were some industrial hubs, such as Delmak, with shipyards. Each System Lord commanded a fleet of Ha'tak-class Motherships, each one capable of wreaking massive damage on a planetary civilization via orbital bombardment with weapons rated at 200 megatons. Very prominent System Lords sometimes fielded larger command ships, but these were quite rare.
Goa'uld ground forces were composed of Jaffa infantry, which -- as stated before -- normally fight in a semi-ritualized form of warfare. The standard Jaffa infantryman wore metallic armor which offered some protection against small arms and wielded a spear-shaped staff weapon capable of firing energy bolts at a rate comparable to that of a semi-automatic rifle. Additional weaponry included more powerful staff cannons, Zat'nikatel sidearms and stun grenades. Their forces lacked indirect artillery, ground vehicle support, camouflage, and night vision equipment, and they used outdated tactics such as massed charges, putting Jaffa at a massive disadvantage against Terran infantry in combat. Airpower in the form of Al'kesh Bombers and Death Glider fighter craft somewhat offset the lack of surface support gear. The Goa'uld were known to supplement their Jaffa forces with levied Humans to serve as cannon fodder, a practice most commonly used by Goa'uld driven to desperation.
One of the biggest weaknesses of the Goa'uld was the fractured nature of their society; the constant infighting was a glaring weakness to those seeking to undermine it (most notably the Tok'ra and the forces of Stargate Command). Loss of face or the exposure of the plans of one particular System Lord involving the violation of an agreement or the development of new weapons technology that could tilt the balance of power in his/her favor to the rest and could easily bring down their wrath upon the offending System Lord. Another flaw was their dependence upon religion and mysticism to keep their Jaffa and human subjects in line; a major effort of Stargate Command was to dispel the notion of Goa'uld divinity.