During a national presidential election, each state sends representatives, members of the Electoral College to vote on behalf of the state's population. Our Constitution provides for the electors as a way of sharing power between the Federal and State governments in our country's system of federalism. This way, neither the government nor the population at large are completely responsible for electing a president.
Each state, plus the District of Columbia, gets a set number of electors based somewhat on population. The number of electors is just the number of Senators (always two) plus the number of Representatives in the House. This does not track proportionally, from state to state, based on population. The numbers are updated every ten years with results from the National Census. For the decade 2000-2010, there are 538 total electors. A presidential candidate must receive a majority of votes from the Electoral College, or 270 votes, to be declared the winner.
While the Constitution provides for such a system, it is not detailed in the methods of carrying it out. The Office of the Federal Register oversees the process of elector nomination, usually at state party conventions, and organizes their voting. Almost all states use a winner-takes-all system, so that electors are pledged to vote for whichever candidate wins the popular vote of the state. Only Maine and Nebraska use proportional systems that might award some electoral votes to one candidate and some to another. In fact, the electors are not legally bound to vote for the leading candidate, but they are usually loyal to their party. If there is no candidate who receives a majority of Electoral votes, the decision is made in Congress, where each state gets only one vote, cast by a Representative.
For almost as long as the Electoral College existed, there has been a debate over its efficacy. Those who'd like to abolish or renovate the system point out that it is possible to win the presidency without winning the national popular vote, which they feel is illogical. Others believe we no longer need such a carefully-guarded balance between "the masses" and a centralized government. Critics also point out that sparsely populated states, since they are guaranteed at least three electoral votes, have an unfair advantage in the disproportional allocation of electors. To make significant changes to the Electoral College, however, would require a Constitutional Amendment.