The Goa'uld are a parasitic alien race which inhabit the Unknown Regions, taking hosts—specifically Tau'ri and Humans—as hosts, and using their advanced technology to pose as Gods for primitive civilisations and enslave entire planets. Infant Goa'uld are incubated in Ja'ffa, a race of warriors bred for this specific purpose, who also worship the Goa'uld as Gods and serve as their military.
Until recently, a large part of the Unknown Regions formed the Goa'uld Empire, ruled by the System Lords, of which there are dozens, who are served by other minor Goa'uld. Following the Ja'ffa rebellion, the Goa'uld Empire has fallen and many of its leaders are either captive or in hiding.
Rogue Squadron has had run-ins with forces of several System Lords, most notably Sobek, Ba'al and Khepri.
Sobek's Domain
A minor Goa'uld whose domain mostly resided on the near edge of the Unknown Regions, Sobek formed an alliance with the renegade Eighth Imperial Fleet to try and conquer the Unknown Regions.
The alliance was believed smashed when their jointly constructed Super Star Destroyer was sabotaged, but Sobek escaped death and took High Admiral Steele has a host, effectively bringing the Imperial fleet into his domain.
He was captured by Rogue Squadron after a double-cross by Khepri, and his domain became the core of Malphuus's Imperial Union.
Khepri's Domain
Khepri's small domain bordered Sobek's, which he saw as a potential target. Khepri made the mistake of taking on the clone Malphuus as his right-hand man, who pursuaded him to make an alliance with Rogue Squadron against Sobek, and then betray them.
The double-cross went badly for Khepri, who was ultimately captured. His domain was integrated into Malphuus's Imperial Union.
Ba'al's Domain
Ba'al was one of the major System Lords following the fall of Apophis. Rogue Squadron encountered his forces several times, once when he tried to capture Sobek's cloning facility for himself, and again over Kurava.
He has not been encountered since the destruction of his Kurava Warship construction yards.