"The Senate of the United States shall be composed of two Senators from each state, chosen by the legislature thereof, for six years; and each Senator shall have one vote."

The US House of Representatives are elected every two years.  And, the election of US Senators are staggered to where 1/3 of senators are up for reelection every two years.

To read the entire article of the US Constitution about the US Senate, please visit the LawDocs section of LawForKids.org.