The House and the Senate both create legislation and vote on it. They are also filled by elected representatives. The House and the Senate also have investigatory powers.
The Constitution originally stated that senators would be selected by legislatures in the states they represent. This was largely due to the authors' reluctance to make the United States too democratic. Changes that occur too quickly, they reasoned, can be counterproductive, and unelected senators serving lengthy terms could prevent bad legislation from passing.
Since the passage of the Seventeenth Amendment in 1913, senators have been directly elected by eligible citizens of the states they represent. Unlike members of the House, however, senators are elected to six-year terms instead of two-year terms. There are also two senators per state compared to a number of representatives based on the state's population, which gives smaller states comparatively more power. Senators have more oversight than their counterparts in the House; presidential appointees must be approved by the Senate before they can serve.
The fundamentals of both the House and the Senate, however, are similar. The meet at the same time, and they often work together when crafting legislation. While certain types of bills can only start in the House, most legislation can start in either body.