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 Governor Romney and President Obama have returned 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.