The US Congress is the legislative branch of the federal government composed of two chambers: the Senate and the House of Representatives. The Senate is one of those two chambers, so "Congress" is the broader term encompassing both bodies, while "Senate" refers to a specific part of it.
The Senate has 100 members—two from each state, regardless of population—who serve six-year terms. Senators represent their entire state. The House of Representatives has 435 members, apportioned among states based on population, serving two-year terms. Representatives serve specific districts within states.
Both chambers must pass identical bills for legislation to become law. However, they have different responsibilities: the Senate ratifies treaties, confirms federal judges and cabinet officials, and tries impeached officials removed by the House. The House initiates revenue bills and votes on impeachments. Senators are typically seen as representing broader statewide interests, while Representatives focus on local district concerns.
The founding structure gave the Senate greater prestige and stability through longer terms and smaller membership, while the House was designed to be more directly responsive to changing public opinion. When people say "Congress passed a bill," they mean both the Senate and House approved it. When they say "the Senate voted," they're referring specifically to that one chamber.