class - Article I, section 3 of the Constitution requires the Senate to be divided into three classes for purposes of elections. Senators are elected to six-year terms, and every two years the members of one class—approximately one-third of the Senators—face election or reelection. Terms for Senators in Class I expire in 2013, Class II in 2015, and Class III in 2011.
Congressional Sessions lists all Congresses and corresponding years - from 1789, when the 1st Congress met, through the current Congress.
You can access legislative information, by bill number or key words, from the THOMAS Web site. Information from the present back to the 93rd Congress (1973) is available on THOMAS.
For information on the Senate, Congress, the legislative process and the federal government, the Virtual Reference Desk is a good place to begin.