SDB gives a great description of gerrymandering, and I'd like to draw attention to my earlier post on the topic, and my observation that the 17th Amendment -- together with modern gerrymandering -- has nearly reversed the roles of the House and Senate.
Consider that before the 17th Amendment, state legislatures selected Senators for their state; members of the House of Representatives were elected directly by the people, but Senators were not. However, with the 17th Amendment and the current state of gerrymandering, the situation has almost reversed itself. State borders cannot be modified, and so Senators are elected directly by the people they represent, while the state legislature fiddles with the Congressional districts and in effect selects the party of the Representative that holds each seat.In the 2000 election, 392 of 399 House incumbents won reelection (98%), but only 23 of 29 incumbent Senators retained their seats (79%). No single election is definitively representative, of course, but I believe these numbers are pretty standard.