Committees decide which bills and resolutions move forward to consideration by the House or Senate as a whole. Committee chairmen have enormous influence over this process.
The joint committees are made up of both senators and representatives. They typically have an oversight or policy role but no legislative duties.
While the Joint Committees on Printing and the Library have nominal responsibility for oversight of the Government Publishing Office and the Library of Congress, respectively, the committees are essentially defunct and oversight takes place within the majority party leadership of each chamber.