Senators are elected for 6-year terms and can be elected to the United States Senate an unlimited number of times. In the initial writing of the U.S. Constitution, Senators had to be elected by state legislatures, but the 17th Amendment gave the power to the people with election by popular vote.
The Senate is the upper-house of Congress, and the lower-house of Congress is the House of Representatives. Together, these houses make up the legislative branch of the United States government. They are responsible for passing legislation, regulating trade and money, declaring war, impeaching federal officials if need be and overriding presidential vetoes (which must have two-thirds of the vote in both houses). The Senate is responsible for approving presidential nominations to federal courts and federal positions, as well as approving treaties.
Each state has two senators and a number of representatives, whose number is determined based on the population in the district. The House of Representatives officials are elected for 2-year terms, which are also unlimited and are elected by popular vote. These officials must be over 25 years of age, have lived in the U.S. as a resident for 7 years and also be a resident of the district that they would represent if they won the election.