http://www.quora.com/Inflation/How-does-printing-money-cause-inflation
One way of looking at Money is that it's a certificate that represents something of real value (food, clothing, services,...)
Each time new value is produced, an equivalent amount of money can be printed to represent it, thus maintaining a constant money/value ratio
However, if money is printed without a corresponding value creation, the ratio of money to value must change - go up