http://www.slate.com/id/2089062/#ContinueArticleOfficial cover. Most CIA employees engaged in operations overseas are given official cover: a sham job in the U.S. embassy (or working for another government agency) that affords them diplomatic immunity. These spies work under varying degrees of secrecy—the CIA station chief in a major ally nation may be well-known on the diplomatic cocktail circuit, but his subordinates, who actually recruit new informants, may not be. Such spies probably confide in their immediate families, but otherwise are unlikely to reveal their true occupation. (Although some operatives working in allied nations are "declared" officers, which means the CIA informs the host government that they are spies.) The advantage of official cover is that if officers are caught, they enjoy the benefits of diplomatic protection; at worst, they'd be publicly outed and sent home in disgrace.
Nonofficial cover. NOCs (the word rhymes with "rocks") are the most covert CIA operatives. They typically work abroad without diplomatic protection (often they pretend to work for some commercial enterprise). If these spies are caught, there's no guarantee that the United States would admit their true identities. When using official cover could put a spy's life and work at risk, NOC is the only alternative.
Why is it such a big deal that someone outed Valerie Plame? For starters, it's a felony. And Plame was also reportedly a NOC with years of experience investigating weapons of mass destruction. If this is true, her discovery could compromise intelligence operations she was involved with around the world, which would explain why she maintained her nonofficial cover even when she was back in the United States. "Hard target" countries like China and North Korea often keep records of every known meeting between Americans and their scientists and officials. Almost certainly, those lists would have been frantically reviewed when Plame's identity was revealed, and any sources she recruited could have been exposed.
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so very few are not covered. It's probably possible for other agencies to discover the "light covered" and even others (they are not completely moronic). But the fact that it would be easy to do it doesn't change the fact that it is a felony to do it. Like saying "your honor, it was so easy to break in that bank so I can't be charged for theft".