A zatarc is a victim of Goa'uld mind-control technology. A mission is programmed into the victim's subconscious and covered with a false memory. The victim is unaware of the programming, which requires only moments of access, but when triggered by a visual or auditory signal, the programming takes over, the victim completes the mission, and then takes his own life.
On a mission to P6Y-325, members of SG-14, including Major Graham and Lieutenant Astor, were captured by the Goa'uld and programmed as zatarcs to assassinate Supreme High Councilor Per'sus of the Tok'ra. The assassination attempt failed, and both took their own lives. Martouf had also been programmed as a zatarc assassin, and he died in his attempt to kill the President during negotiations with the Tok'ra leader.
Zatarc programming is fairly recent technology, and the early experimental zatarc detector developed by the Tok'ra has been gradually modified as a viable means to counteract the programming. Goa'uld brainwashing techniques have also continued to evolve, and Ba'al used similar brainwashing technology on Teal'c and members of the Jaffa High Council in a failed attempt to gain control of the Jaffa Nation, as well as on Agent Barrett of the NID, who was programmed to provide Ba'al with a means of escape from the SGC. With time and dedication, the effects of the brainwashing can be overcome.