Senate and the House of Representatives. 'The House' is the bigger body because congressmen are elected at a local level while the Senate consists of two senators from each State.
At present the Presidency & the Senate are controlled by the Democrats. The Republicans have a majority in the House. Legislation has to pass from the House to the Senate and finally to the President.
Because the two main parties (the Republicans & the Democrats) have major philosophical differences (listen to a short explanation here) most legislation is contested.
Basics of American Politics (14th Edition)
The Complete Idiot's Guide to U.S. Government and Politics