"The Speech or Debate Clause (Clause) of the U.S. Constitution states that '[F]or any Speech or Debate in either House,' Members of Congress (Members) 'shall not be questioned in any other Place.' The Clause serves various purposes: principally to protect the independence and integrity of the legislative branch by protecting against executive or judicial intrusions into the protected legislative sphere, but also to bar judicial or executive processes that may constitute a 'distraction' or 'disruption' to a Member's representative or legislative role. Despite the literal text, protected acts under the Clause extend beyond 'speeches' or 'debates' undertaken by Members of Congress, and have also been interpreted to include all 'legislative acts' undertaken by Members or their aides."
CRS Report for Congress, R45043
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html