For a more technical explanation, check this out:  http://www.friesian.com/money.htm

 

Excerpt:

 

"... What the value of money actually is (i.e. what units of the standard will buy, in general) depends on 1) how much money there is, 2) how much money is held out of circulation, and 3) how many exchanges circulating money is used to cover. This is the "quantity theory of money" and can be expressed in a famous equation by the American astronomer and economist Simon Newcomb:  MV = PT.  "M" signifies the actual quantity of money; "V" signifies the "velocity," which is the rate at which money circulates or how long money is held out of circulation; "T" is the number of transactions, or exchanges; and "P" is the level of prices. This equation easily illuminates most questions about inflation or deflation, which is how money becomes less or more valuable over time. The evidence for the "quantity theory" is that historically inflation and deflation have occurred independently of economic growth and recession, as can be seen in the data from Friedman and Schwartz given below.  ..."