The Cult of Sokar was an information and intelligence network developed by the Goa'uld Sokar in order to spy on the System Lords. Its ranks consisted of spies and other operatives that were spread across the galaxy where they served virtually every single Goa'uld. The cultists consisted of both Jaffa and Humans as well as at least one Goa'uld along with a number of Unas that operated on more primitive worlds. They were also the most devoted and dedicated of followers as well as worshiped their Lord Sokar to the point that they reveled in his demonic ways. As such, the Cult of Sokar actively worked to further his agenda.
Whilst they were mostly used to gather information on his rivals and enemies, Sokar was known to have used the Cult for other missions such as sabotage or even assassinations. These newer mission roles were typically one way as the cultist was never expected to survive them even if they were successful and these individuals that participated in such operations did so gladly as they believed it to be part of a sacrifice. In their primary role as informants, the cultists typically listened to any pertinent information that would be of use to their Lord. When they did discover something, they usually passed this knowledge to their comrades until it eventually reached the ears of Sokar. On some occasions, their reports took the form of communication spheres or recording devices whilst on other moments the cultist were required to remember what they had seen or heard. Any false information led to the cultist suffering eternal damnation at the hands of Sokar - a fate that was worse than death.
Though seen as a spy network, the Cult of Sokar was more akin to an actual religious body that worshiped their Lord and Master. Being fanatics to their cause, the cultists often performed sacrifices as well as bizarre rituals and were active in the recruitment of others into their organization. Ultimately, these individuals sought to serve Sokar and believed that when he ruled the galaxy - they would become his trusted servants and rule as lords of his domain. (RPG: "Living Gods: Stargate System Lords")