http://www.cnn.com/2004/ALLPOLITICS/01/22/cia.leak/
Basically, there was this guy who wrote a column critical of the Bush administration's handling of Iraq. Then, suddenly, someone released his wife's name as a covert CIA agent, basically ruining her life and career (Also, it's a federal felony to do that in the US). This leak was released to several columnists, and they published it in newspapers. The husband accused the whitehouse for releasing this info as retribution, and the administration testified (including Rove) in front of a Grand Jury that they were not involved. If it's true that Rove was involved, then he's committed Perjury as well as a federal felony with a possible sentence of 10 years in federal prison.
If the authorization to do this goes higher than Rove, then some very important people are screwed.