According to About Congress.gov:
The Congress.gov beta site contains legislation from the 107th Congress (2001) to the present, member of Congress profiles from the 93rd Congress (1973) to the present, and selected member profiles from the 80th through the 92nd Congresses (1947 to 1972). Over the next two years, Congress.gov will be adding information and features, eventually incorporating all of the information currently available on THOMAS.gov.
What's New in Congress.gov?
•Ability to simultaneously search all content across all available years
•Ability to narrow and refine search results
•Easier identification of bill progress/status
•Links to House and Senate floor video, top searched bills and save/share features
•Updated, easy-to-understand page designs for bills, members, and related items
•Clean search results display
•Responsive design for mobile devices
•Consistent, meaningful and permanent URLs
Each piece of legislation has a landing page with tabs to help users find key information for the bill including Summary and Text, Major Actions, Titles, Amendments, Cosponsors, Committees and Related Bills.
What Is the Timeline for New Releases to Congress.gov?
This initial launch of the beta site includes bill status and summary data, as well as bill text from the 107th Congress (2001) to the present, member profiles from the 93rd Congress (1973) to the present, and some member profiles from the 80th through the 92nd Congresses (1947 to 1972).
Following the initial beta phase, subsequent releases will make additional data available, including the Congressional Record, Congressional reports, the Congressional Record Index, House and Senate Calendars, nominations and treaties.