Eric Carter is an agent with CTU.
That acronym stands for the Counter Terrorism Unit. When the nation suffered the 9/11 attack, the CTU was created under the control of Homeland Security. Located in Los Angeles, or at least one section of it, it is tasked with ferreting out terror cells on the west coast and keeping the country safe. The latest in technology is employed by a highly skilled team with most of them operating in a command center using street cameras, building security monitors, and satellite imagery to hone in on the activities of those the CTU suspect are up to no good or of those field agents tasked with doing the actual stopping of those bad acts.
We first meet Carter when he is the NCO of a squad of U.S. Army Rangers who are sent into Yemen to find and capture, if possible, notorious terrorist leader Ibrahim Bin-Khalid. In a major fire-fight, that wanted leader is killed and after a short time, the team is returned to the States. When it is learned that the dead man's followers have issued a fatwa against all the team members, they are placed in Witness Protection for their safety. Carter and his wife Nicole are moved to the West Coast for their placement.
It is when Bin-Khalid's men somehow find and attack Carter and Nicole that Carter will be forced to join up again with Rebecca Ingram, the former CTU leader who had choreographed the initial Yemeni raid. This is how Carter will become, at least for the 24 hours it will take to resolve a very messy situation, a CTU operative.
We learn in another story how a teenage Carter grew up in the southeast side of Washington, D.C. along with his brother, Isaac. It is Isaac who comes up with the idea of stepping up his penny-ante drug selling with a major score and Eric is pulled in into. It is there that Eric will be "forced to become a soldier long before he ever joined the army". In doing so, he also won the heart of Nicole.