The Electoral College is the system that elects the president of the United States. Each state is allotted electors based on the size of its congressional delegation. Electors from each state vote for the president based on the popular vote winner in their state, with two exceptions, and the winner of the electoral college vote is declared president.
The Electoral College is designed to keep smaller states and rural areas from being overshadowed by larger states and larger cities.
There have been rare cases in which the Electoral College winner did not win the popular election. In 2000, George Bush won the Electoral College vote and became president, even though he did not win the popular vote nationwide.
Each state gets a vote for each of its two senators and one for each of its congressional districts. For example Tennessee has nine districts and two senators, so it gets 11 electoral votes.
In all but two states, the popular vote winner gets all of the electoral votes for the state. Nebraska and Maine award the two Senate votes to the popular vote winner and each district vote goes to the candidate who won that district. States may make their own rules in this regard.
The Electoral College is made up of people appointed by the political parties. These people meet at the state level to cast their votes officially. They are not legally required to vote for the candidate they were assigned to vote for. Very rarely has this happened, and it has never affected an election.
If there is a tie in the electoral vote or if no one gets the required 270 votes, the election is decided by the House of Representatives. In the House, the delegation of each state would have one vote to cast for president and the winner there would be named president.