Today, the two houses of Congress, the House of Representatives and the Senate, are broadly similar, and they both craft and vote on legislation. The House must initiate budget-related bills, and the Senate has oversight for presidential nomination.
Both the House and the Senate must approve bills before they become law, and they are both composed of elected representatives. While senators were originally chosen by state legislatures, this changed in 1913 when the Seventeenth Amendment was enacted.
The House must initiate budget-related laws. However, it is not entirely clear which laws can only be initiated by the House, and those interpreting the Constitution have taken a favorable approach to letting the Senate start legislation. Laws dealing with the national budget, however, always start in the House. The Senate, on the other hand, has oversight on presidential nominations, including Supreme Court nominees.
The authors of the Constitution initially wanted the House of Representatives to be the "lower house" and the Senate to be the "upper house," which is similar to England's parliamentary system. While elected representation was valued, the authors were loathe to allow citizens to choose all lawmakers. The Senate, in their minds, would help prevent a populist frenzy from passing dangerous laws.