Why were the mid-term elections important?
At stake was control of Congress, the legislative branch of the US Government. Republicans had controlled both chambers - the House of Representatives and the Senate - since 1994, except for a brief time when Democrats held the Senate after one senator defected from the Republican party.
All 435 House of Representatives seats were at stake, while one-third of the 100 Senate seats were up for re-election.
The Democratic Party has taken control of both the House of Representatives, the larger of the two houses, and the Senate.
The party seized the Senate after narrow wins in two key races (Virginia and Montana).
That assumes that the two Senators elected as independents (Joe Lieberman in Connecticut and socialist Bernie Saunders in Vermont) vote with the Democrats.
However, the new 110th Congress will not take office until January 3, 2007.
The result represents a striking rejection of the policies of Republican president George W Bush, who still has two years remaining in office.
They were also a reflection of public disillusionment with a series of scandals that have rocked the Republicans in Congress.
What does this mean for politics?
Under the US system of government, with its separation of powers, the Congress proposes and passes legislation, which the President has to implement.
There have often been times in which control of the Presidency and control of Congress have been held by different parties, such as during much of the Clinton administration in the 1990s.
If the president and Congress are prepared to work together on a bi-partisan basis, then legislation can still be passed.
But if the two sides fundamentally disagree, the result is often deadlock.
The president can block legislation by using his power of veto, which can only be overturned in Congress by a two-thirds majority.
However, he usually will want to use this power sparingly.
Mr Bush has only used the veto once during his term in office.
What is the role of the Speaker of the House, Nancy Pelosi?
The Speaker of the House is the presiding officer in the House of Representatives, with the power to recognise speakers, call for votes on legislation, and appoint members of committees.
The Speaker is also the second in line of succession for the presidency, after the vice-president.
If elected in January, Nancy Pelosi will be the first female speaker of the House.
However, the Speaker's role is also a partisan one, and Ms Pelosi will also be the leader of the Democrats in the House, supported by the House majority leader, Steny Hoyer, who is the "floor leader" whose job it is to line up votes for legislation backed by the leadership.
Ms Pelosi will also share the role as the effective leader of the Democrats with Harry Reid, the Senate Democratic leader, and to a lesser extent with Howard Dean, the leader of the Democratic National Committee.
What about foreign policy?
Mr Bush will still have the power to set US foreign policy, and will still be the commander in chief of all US military forces.
But he will be subject to the "advice and consent" of the Senate, who must approve treaties and appointments, such as ambassadors.
Congress can also use its oversight powers by holding hearings on key issues in foreign policy, and calling administration officials to testify.
And it still holds the purse-strings, and can increase or decrease the military budget or spending on homeland security.
However, Mr Bush's appointee as defence secretary, Robert Gates, who is a member of a bi-partisan commission charged with coming up with a new strategy in Iraq, may signal a new approach to foreign policy.
In most of the post-war era, presidents were careful to build cross-party support for their foreign policy.
What about domestic policy?
There are still big disagreements between the administration and the Democrats on many key areas of domestic policy, such as tax cuts.
The Democrats also want a different approach to tackling health care, energy policy (including more emphasis alternative energy sources), and domestic security.
But there may be a bi-partisan attempt to tighten up the rules on ethics and the role of lobbyists in Congress, following the Abramoff scandal.
And there may be a more bi-partisan attempt to tackle the budget deficit and immigration.
However, there are big differences on many of these issues within the Democratic party itself.
And even when controlled by the same party, the House and Senate do not always agree on their approach - so there could still be considerable difficulties in passing major legislation.
What about the social agenda?
Another big disagreement between Mr Bush and many Democrats has been the so-called "values" agenda - issues such as abortion, gay marriage, and stem cell research.
Mr Bush may face attempts in Congress to change course on some of these issues now that the Democrats are in control.
Some state-wide referendums held at the same time as the mid-terms delivered a rather mixed message about voter views on these issues.
Voters in seven states - Virginia, South Carolina, Wisconsin, Tennessee, South Dakota, Colorado and Idaho - rejected gay wedlock by limiting marriage to unions between men and women.
But in South Dakota, voters overturned a near-total ban on abortions passed by the state legislature earlier this year.
And in Missouri Democrats and liberals were claiming another victory after the passage of a state constitutional amendment enshrining the right to conduct embryonic stem cell research, something opposed by President Bush.
However, many of the newly elected Democratic Congressmen and Senators come from the more socially conservative states, and a number of them have expressed support for the "values agenda" themselves.