What is the electoral college?States send delegates to the electoral college according to the state vote - with 48 out of 50 using a 'winner takes all system'. Only in Maine and Nebraska are the delegates allocated proportionally.
The number of delegates sent to the electoral college is decided by the population of the state. Thus the largest delegations come from the most populous states: New York, California and Texas.
Most states are comfortably red (Republican) or blue (Democrat) - see here. This means that presidential elections are essentially fought over a small number of purple or battleground states which swing between parties in different election cycles.
Ohio is the classic example of a swing state. It has voted for the victor in every election since Kennedy's victory in 1960. Traditional wisdom suggests that the Republicans cannot win without Ohio - the state which decisively swung the re-election of George W Bush in 2004. That is why presidential candidates return again and again to campaign there.
One curious feature of the electoral college system is that a candidate can win the popular vote but still become president - this was the case in the 2000 election. The distribution of votes is as important as the raw number.