When the framers of the Constitution met in Philadelphia, they wanted a government that truly represented the people. Unfortunately, they couldn't decide exactly how to do this. There had been very few democracies before this, so they didn't have a lot to go off of. Through the Great Compromise they decided on the bicameral legislature (two houses). To find out more, scan through the links below. I especially recommend the Acts of Congress. As always, email me if you have any questions.
House of Representatives
Acts of Congress
Athens had a direct democracy, but that meant that every citizen directly influenced every law. Most delegates felt that the typical American was not intelligent enough to do this, and besides, thousands of people would have to attend. The problem would only get worse if America gained new territory.
New Jersey Plan
The New Jersey plan, introduced by William Patterson, favored the small states. It provided that every state should have a set number of representatives, and that number should be equal for all states. Of course the question was, "Is this fair?" The large states didn't seem to think so.
Edmund Randolph put forward the Virginia Plan, which favored the smallers states. This one was based soley on population. The larger the population, the more representatives you had. The number of representatives would change with every census.
The large states felt their rights would be trampled on by the Virginia Plan, and the large state felt that the New Jersey Plan didn't serve a democracy. The Great Compromise combined the two into a bicameral legislature, and that is what we still have today.