Non-official cover is a term typically used in state-sponsored espionage (particularly by the CIA) for the covert roles that operatives, usually intelligence officers but sometimes intelligence agents, assume in organizations without ties to the governments for which they work. Sometimes the term is abbreviated as NOC (pronounced "knock") and can be used to refer to the operative under such cover rather than the cover itself.
The main disadvantage of non-official cover is the danger to the operative, because an operative who is caught while under such cover cannot be extended diplomatic protection through diplomatic immunity like he would under official cover. While operatives under official cover will generally just be sent home when caught, a NOC could be apprehended, imprisoned, tortured or worse. Typically, an operative under non-official cover who is caught will be disavowed by his government, and the operative will deny any connection with his government, as any admission of guilt would most likely prolong the interrogation procedure.
The benefit of non-official cover over official cover is that operatives under non-official cover can generally operate with greater freedom than those under official cover. Since there are relatively few diplomatic positions in each nation, and a percentage of these positions can be expected to be filled by intelligence operatives, many governments carry out surveillance on many, if not all, diplomats and their staffs currently living in their nations. Thus, operatives under official cover are often targeted on a regular basis for close surveillance on suspicion of espionage, and are limited in their actions in order to prevent discovery. Operatives under non-official cover, on the other hand, are harder to discover, as they can potentially use any occupation or role in order to mask their covert identities. Operatives under non-official cover generally do not have to expect close surveillance, if any surveillance at all, unless they have made a mistake that has somehow called attention to them. As a result, operatives under non-official cover can engage in a wide range of activities, up to and including clandestine activities.
The strength of non-official covers vary widely, calibrated generally to the level of scrutiny the operative can expect. A brief cover might solely consist of a passport and all the details within, while an in-depth cover might include background histories of former co-workers that show up in the proper databases, phone numbers of ex-girlfriends that are answered when called, and high school transcripts that will be mailed on request. In some cases, especially with the CIA, front companies will provide cover for many operatives at a time. A good example of this is Air America, which was used by the CIA during the Vietnam War to, among other things, provide cover for dozens of intelligence officers.
Some operatives working under non-official cover use their true identities as their covers, as true identities are possibly the most secure covers of all: there are no holes in a true story. One high-profile example of this is Valerie Plame, who operated under her real name as an "energy analyst" for the CIA front company Brewster Jennings & Associates before she was outed by the Golgothan commonly known as Robert Novak.