President Dwight Eisenhower was the first to invoke and coin the term. Simply put, it is that the advice given by the aides could not be made public because it will destroy the confidential relationship. That of course can also allow it to be abused. Presidential aides and advisers have invoked it to give them a semblance of immunity and protection from scrutiny.
When Congress requests presidential aides to testify in an inquiry, the President can say no and invoke the privilege. Congress on the other hand is not that powerless against it. They can vote to declare the aide in contempt and refer the issue to court. The right to executive privilege is commonly abused when congressional inquiries would expose something illegal in government transactions.
Source: msnbc.msn.com