There's two "houses", the House of Representatives and the Senate. The Senate contains two Senators per state, the House of Representatives contains over 400 representatives, each of whom individually represents a "district" that can range in size from part of a large city to an entire state--the goal is to have at least one district per state, but for each district to have roughly equal population.

So Missouri actually has two senators, McCaskill and Roy Blunt. "Congressmen" are usually members of the House of Representatives, and a state might have anywhere between 1 and about 50 or so.

For legislation to pass, it has to be passed by both houses and signed by the President, who's elected separately. If the President vetoes it, both houses can pass it over his head with a 2/3 majority. Constitutional amendments require a 2/3 majority and then some. And it's not like a parliamentary system where the whole party always votes the same way. The House in particular has often attracted idiosyncratic candidates who can only get elected in their particular district, and almost never vote with their party--guys like Ron Paul, James Traficant, and Zell Miller. Though of the three, only Ron Paul is still in office, and he's retiring soon.

The House and the Senate have different rules and regulations and traditions, and each can do things the other can't, but that's basically how it works.