We live in an economy with a certain amount of economic activity and overall wealth, which tend to go up over time (barring recession). To facilitate trade, the central banks of nations issue currency, which acts as a "dual" of the economy (or more precisely, a piece of it - there aren't enough dollars circulating to buy every good and service in the US simultaneously), a pure trade good with fiat value. If the economy kept growing and the supply of currency stayed constant, deflation would result, as previously noted. However, by Keynesian theory (which is not absolutely proven be right, but has worked pretty well over the past 75 years), inflation (at reasonable levels) is more conductive to investment and consumption and overall economic vitality than deflation, due to people being more inclined to spend and invest money that would lose value just sitting there.
Thus, the government prints more money every year than would be necessary to match the growth in the economy. This is not some ridiculous plot by the Swiss and the Jews to enslave us all; it's simply a tax on everyone who holds dollars (or, in other countries, other currencies), since the government dilutes the overall pool of currency to grant itself spending money. This "inflation tax" is always a very small percentage of overall government revenues in the US, and even if Keynesian theory were totally wrong, the policy would not be particularly harmful. It would just be another tax, although somewhat regressive, since the poor tend to have more of their net-worth in cash than the rich.
And, it makes absolutely no difference who gets the money straight from the printing press, aside from the value of enjoying crisp, shiny new bills. The government spends money it got from printing in exactly the same places as it spends money procured from taxes.
I understand what psychological processes drive people to conspiracy theory, but it still frustrates me to no end that so many people believe in the laws of cigar-munching cabals instead of those of economics.