The main responsibility of the congress is to make laws in both houses while that of the senate is to elect the vice president. The senate is a member of the congress which consist a number of representatives. You can learn more on http://independenceave.org/congress/.
Their purpose can be broken down into three parts: 1) is to pander to lobbyists and large corporations. 2) To blackmail each other to get pork barrel projects slipped into important bills. and finally 3) To think up laws and rules that will never affect them, so they can justify a paycheck. That about covers it.