Although not mandated in the Constitution, the parties in the Senate choose a member who will act as spokesperson. The spokesperson for the party that holds the majority is known as the majority leader, and the spokesperson for the party that holds the minority is known as the minority leader.
Source
Mike Mansfield http://upload.wikimedia.org/wikipedia/en/8/89/Mike_mansfield.jpg; LBJ http://www.loc.gov/wiseguide/aug03/johnson.html; George Mitchell http://en.wikipedia.org/wiki/Image:George_J._Mitchell.jpg;. Created by Gary Keith and Mike Frost. Tom Daschle http://en.wikipedia.org/wiki/Image:Daschle.jpg; Howard H. Baker, Jr. http://en.wikipedia.org/wiki/Howard_Baker; Bob Dole http://en.wikipedia.org/wiki/Image:Bob_Dole.jpg; Trent Lott http://en.wikipedia.org/wiki/Image:Trent_Lott_official_portrait.jpg; Bill Frist http://en.wikipedia.org/wiki/Image:B_Frist.jpg.