“(4) The term “covert agent” means- (A) a present or retired officer or employee of an intelligence agency or a present or retired member of the Armed Forces assigned to duty with an intelligence agency-
“(i) whose identity as such an officer, employee, or member is classified information, and
“(ii) who is serving outside the United States or has within the last five years served outside the United States; or”
The remaining category of employee is covert. Covert employees include people who work under “official cover” and people who work under “non-official cover”. A former CIA officer, Tom Gilligan, discussed both types of cover in his book CIA Life: 10,000 Days With the Agency. Official cover means the employee can say that he or she works for the United States Government, e.g. State Department, but at no time do you admit publicly that you work for the CIA. You get the added benefit of carrying an official or diplomatic passport. If you get caught overseas engaged in intelligence activity it means you have diplomatic immunity and the equivalent of a get out of jail free card.
Non official cover or NOC also is covert but is more sensitive (and dangerous). A NOC does not work for the U.S. Government. A NOC does not have an official or diplomatic passport. A NOC works for a business or organization with no tie to the U.S. Government. If you are caught overseas while conducting espionage activities as a NOC you are screwed. You do not get a jail out of free card. You remain in jail or may be executed.
http://fighting-dems.us/Newsletter/2007/03/01/was-she-covert/