Some of my friends from India were not aware of how exactly elections in US work. It's different from how it works in India. So here I try to explain the process in simple terms.
Everybody votes, and whoever gets the most votes becomes president, right? Nope. That method, called the Popular Vote is not used to elect the President of United States. Instead an indirect system called the Electoral College is used to elect the president. This isn't really a national election -- it's a whole bunch of separate state elections.
Each state gets a certain number of electors (Electoral Votes); one for each senator (which means two, because there are always two senators) and one for each representative (which depends on the state's population as determined by the census). So in almost all cases, whichever candidate wins a given state wins all of that state's electoral votes, and it takes a majority of the electoral votes to win the overall election.
For example, Florida has 27 electoral votes. Just say Bush wins 15 of those 27 votes, then he wins the state and all the 27 electoral votes goes to Bush. Got it?
There are total 538 electoral votes. A candidate must receive 270 of the 538 votes to win the election. So, while winning the popular vote may not ensure a candidate's victory, a candidate must gain popular support of a particular state to win the votes in that state.
Some states like Utah, Wyoming, Idaho mostly votes for the Republicans. But some states like Florida, Iowa, Wisconsin etc are called swing states (also referred to as battleground states). Swing states are ones in which recent presidential elections have been decided by a narrow margin -- in some cases, less than one percent. That's why you see the candidates, Bush and Kerry, campaigning in these states often. The goal of any candidate is to put together the right combination of states that will give him or her 270 electoral votes. Hope I didn’t confuse more :-)