Originally, the framers intended to have the House be focused on more pressing, everyday concerns, while the Senate would be the more deliberative, policy-centric body. However, these distinctions have generally blurred over the decades since, and now the two houses hold the same amount of power, and essentially have the same duties.
That said, the Senate does play a unique role in the functioning of the U.S. government. For example:
Impeachment: While the House of Representatives initiates impeachment proceedings against government officials, including the President, it is the Senate that investigates the charges and tries the cases against the officials, effectively acting as a prosecutor and jury. Since 1789, the Senate has tried 17 federal officials, including two presidents.
Cabinet, Ambassadorial and Judicial Nominations: The President has the power to appoint members of his cabinet (including secretaries to the various agencies of the federal government), U.S. ambassadors to foreign countries and the United Nations, and justices of the Supreme Court and other federal judges. However, the Senate holds the power to vet and approve these appointments. Appointees who fail to receive Senate approval cannot assume their posts.
Treaties: While the President holds the power to negotiate and make treaties with foreign governments, the Senate must ratify these agreements, and the body does hold the power to amend treaties as it deems necessary.
Censure and Expulsion: Article 1, Section 5 of the U.S. Constitution gives both houses of Congress the right to punish members for “disorderly behavior.” In the Senate, members can be “censured” (a formal term essentially meaning condemnation or denouncement), which is a formal disapproval. The Senate, by a two-thirds majority, can also vote to expel a member for disorderly conduct, a far more severe punishment. Since 1789, the Senate has censured nine members and expelled 15.
Filibuster and Cloture: The procedure known as filibuster—essentially open debate used to delay or block a vote on legislation—has been employed numerous times throughout history. In 1957, Senator Strom Thurmond famously filibustered for more than 24 hours in an attempt to delay a vote on the Civil Rights Act of that year. His filibuster included a full reading of the Declaration of Independence. Since 1917, with the passage of Rule 22, the Senate can vote to end a debate with a two-thirds majority, in a procedure known as cloture. In 1975, the Senate modified the cloture rule to enable enacting of the tactic on a three-fifths majority (60 of the 100 members).
Investigations: Both houses of Congress can conduct formal investigations of wrongdoing on the part of the Executive Branch (the president and/or his cabinet) as well as other officials and agencies. One of the most famous Senate investigations involved the Watergate scandal, which led to the impeachment of President Richard M. Nixon in 1974.
Contested Elections: The Constitution also gives each house of Congress the power to be the judge of the “elections, returns and qualifications of its own members.” Since 1789, the Senate has developed procedures for judging the qualifications of its members and settling contested elections.