From https://en.wikipedia.org/wiki/United_States_Senate

The United States Senate is the upper chamber of the United States Congress, which along with the United States House of Representatives—the lower chamber—comprise the legislature of the United States.

Some information about senate.

• Each state has two senators.
  • Since there are 50 states, there are 100 senators.
• From 1789 until 1913, Senators were appointed by legislatures of the states they represented; following the ratification of the Seventeenth Amendment in 1913, they are now popularly elected.
• Power:
  • The ratification of treaties and the confirmation of Cabinet secretaries, Supreme Court justices, federal judges, other federal executive officials, flag officers, regulatory officials, ambassadors, and other federal uniformed officers.
  • In cases wherein no candidate receives a majority of electors for Vice President, the duty befalls upon the Senate to elect one of the top two recipients of electors for that office.
  • Conducting trials of those impeached by the House.
• Senators serve terms of six years each.
  • The terms are staggered so that approximately one-third of the seats are up for election every two years.
  • both seats from a given state are not contested in the same general election
    • Except a mid-term vacancy is being filled.
• Elections
  • on the first Tuesday after the first Monday in November in even-numbered years.
• Mid-term vacancies
  • mid-term vacancies in the Senate be filled by special election.
  • The Seventeenth Amendment also allows state legislatures to give their governors the power “to make temporary appointments until the people fill the vacancies by election as the legislature may direct”.
  • If a special election for one seat happens to coincide with a general election for the state’s other seat, each seat is contested separately.
  • A senator elected in a special election takes office as soon as possible after the election and serves until the original six-year term expires.