The United States Congress is the legislative branch of the federal government and consists of two houses: the lower house known as the House of Representatives and the upper house known as the Senate. The words "Congress" and "House" are sometimes colloquially used to refer to the House of Representatives.
Democrats currently have a majority in the Senate and Republicans control the House.
edit Seats in the Senate vs House of Representatives
While there are 100 seats in the Senate (two from each state), there are 435 seats in the House of Representatives, one each from the various congressional districts (the number of congressional districts in each state is determined based on the population).
edit House vs Senate Terms
Senators are elected for a six-year term but House Representatives only get a two-year term before they need to seek re-election.
Here's Bill O'Reilly offering an explanation as to why the terms are of different lengths:
edit Two Senators per State
The rule that each state should have equal representation in the Senate (two senators) irrespective of how small or populous the state is, is not universally popular. This article in The New Yorker dissects it well:
James Madison and Alexander Hamilton absolutely hated the idea that each state should be entitled to the same number of senators regardless of size. Hamilton was withering on the topic. “As states are a collection of individual men,” he harangued his fellow-delegates at the Constitutional Convention in Philadelphia, “which ought we to respect most, the rights of the people composing them, or of the artificial beings resulting from the composition? Nothing could be more preposterous or absurd than to sacrifice the former to the latter.”
"House of Representatives vs Senate." Diffen.com. Diffen LLC, n.d. Web. 23 Aug 2014. < http://www.diffen.com/difference/House_of_Representatives_vs_Senate >