In recent light of Obama's assertion of executive privilege over important documents concerning the Fast and Furious operation, here's a brief article concerning this controversial power.
Executive privilege is basically someone saying: "No, I will not release this information". In more wordy terms, it's a power claimed by members of the executive branch (President, VP, etc) to resist interventions or inquisitions into actions or for information.
As you see, it has a broad and vague definition, and it has been used in many controversial ways. It is also not mentioned in the constitution, but is accepted by the Supreme Court as a means of Separation of Powers.
The Clinton administration invoked executive privilege on fourteen occasions.
In 1998, President Bill Clinton became the first President since Nixon to assert executive privilege and lose in court, when a Federal judge ruled that Clinton aides could be called to testify in the Lewinsky scandal.
The Bush administration invoked executive privilege on six occasions.
Bush first asserted executive privilege to deny disclosure of sought details regarding former Attorney General Janet Reno, the scandal involving Federal Bureau of Investigation (FBI) misuse of organized-crime informants
"....Operation Fast And Furious, the illegal, dangerous, and idiotic ATF plan to let guns "walk" across the border into the hands of Mexican drug cartels, allegedly to allow the guns to be tracked back to subsequent criminal activity and then apprehend those responsible"(Article Here)
Obama therefore, asserts executive privilege to withhold documents containing important information on this operation.