Why do we have two chambers in Congress, the House and Senate? Since members of both are elected by, and represent the people, wouldn't the lawmaking process be more efficient if bills were considered by only one body?

While it may appear clumsy and often overly time-consuming, the two-chamber or "bicameral" setup of Congress works today exactly the way a majority of the Founding Fathers envisioned in 1787. Clearly expressed in the Constitution is the Founders' belief that power should be shared among all units of government. Dividing Congress into two chambers, with the positive vote of both required to approve legislation, is a natural extension of the Founders' concept of employing "checks and balances" to prevent tyranny. The Founding Fathers explain the formation of Congress to the people in the Federalist Papers 52-66.

Why are the House and Senate so Different?
Have you ever noticed that major bills are often debated and voted on by the House in a single day, while the Senate's deliberations on the same bill take weeks? Again, this reflects the Founding Fathers' intent that the House and Senate not be carbon-copies of each other. By designing differences into the House and Senate, the Founders assured that all legislation would be carefully considered, taking both the short and long-term effects into account. 

Why are the Differences Important?
The Founders intended that the House be seen as more closely representing the will of the people than the Senate.
To this end, they provided that members of the House - U.S. Representatives - be elected by and represent limited groups of citizens living in small geographically defined districts within each state. Senators, on the other hand, are elected by and represent all voters of their state. When the House considers a bill, individual members tend to base their votes primarily on how the bill might impact the people of their local district, while Senators tend to consider how the bill would impact the nation as a whole. This is just as the Founders intended. 

All members of the House are up for election every two years. In effect, they are always running for election. This insures that members will maintain close personal contact with their local constituents, thus remaining constantly aware of their opinions and needs, and better able to act as their advocates in Washington. Elected for six-year terms, Senators remain somewhat more insulated from the people, thus less likely to be tempted to vote according to the short-term passions of public opinion. 

By setting the constitutionally-required minimum age for Senators at 30, as opposed to 25 for members of the House, the Founders hoped Senators would be more likely to consider the long-term effects of legislation and practice a more mature, thoughtful and deeply deliberative approach in their deliberations. Setting aside the validity of this "maturity" factor, the Senate undeniably does take longer to consider bills, often brings up points not considered by the House and just as often votes down bills passed easily by the House.

A famous (though perhaps fictional) simile often quoted to point out the differences between the House and Senate involves an argument between George Washington, who favored having two chambers of Congress and Thomas Jefferson, who believed a second chamber to be unnecessary. The story goes that the two Founders were arguing the issue while drinking coffee. Suddenly, Washington asked Jefferson, "Why did you pour that coffee into your saucer?" "To cool it," replied Jefferson. "Even so," said Washington, "we pour legislation into the senatorial saucer to cool it."

Constitutional References in this Article
Creation of Congress - The Legislative Branch: Article I
The House of Representatives: Article I, Section 2
The Senate: Article I, Section 3