A system lord is a Goa'uld ruler who presides over a particular area of space. System lords may maintain starships to preside over their domain, and they usually use Jaffa warriors (who bear the symbol of their lord on their foreheads) as soldiers. Many system lords obtain tribute from worlds they control, often in the form of mining operations that result in valuable ore. Many of the system lords mentioned in Stargate SG-1 are gods and goddesses of Earth myth: Ba'al, Osiris, Ra. Those whom they rule consider the system lord to be their god.
The system lords maintain an uneasy peace with each other. They have a treaty with the Asgard, maintained mostly by the threat of the Asgard's superior firepower, which the system lords skirt and test. One criticism of Earth's Stargate program is its tendency to kill off system lords, thus destabilizing the power structure without really fundamentally changing anything. Remaining system lords then scramble for territory and power.
Although presumably system lords exchange their protection for the labor and tribute provided by the planets they control, in practice, we haven't seen much of this. System lords seem far more likely to abandon planets, the inhabitants of which may wait in vain, sometimes for centuries, for their god to reappear.
--Kylie Lee 11:43, 9 Aug 2004 (PDT)