http://en.wikipedia.org/wiki/United_States_CongressArticle I of the Constitution states "all legislative powers herein granted shall be vested in a Congress of the United States, which shall consist of a Senate and a House of Representatives." The House and Senate are equal partners in the legislative process—legislation cannot be enacted without the consent of both chambers. However, the Constitution grants each chamber some unique powers. The Senate ratifies treaties and approves presidential appointments while the House initiates revenue-raising bills. The House initiates impeachment cases, while the Senate decides impeachment cases
I don't have time to look for a better source, but this is how I understood the role of the Legislative Branch.