The US is not really a functioning democracy right now so it’s kind of a crapshoot to be discussing what they do now, however if you’re interested more in the idea of what they’re supposed to do, they propose and vote on new legislation, same as the members of the house of representatives. Generally, bills can be proposed by either group, and both bodies must pass the bill for it to reach the president to be signed into law.

What makes the two different is that the senate is intentionally much smaller, and all states are equally represented (2 senators each), unlike the house which is proportionally represented with more populous states having far greater numbers of representatives and thus votes. The senate also changes members less frequently with senators being elected for 6 year terms compared to the president’s 4 years and the representatives at 2 years. The idea for the senate is for it to be responsible for considering longer term effects of proposed legislation, as well as to consider and propose legislation on issues that affect the whole nation equally including the smaller states that have less of a voice in the house of representatives. In theory, it’s a very thoughtful and well-balanced system. The reality is… well, we can all see what’s happened to it.