The FOMC is an acronym for the Federal Open Market Committee.

The FOMC meets eight times a year to set key interest rates, such as the discount rate. They also makes decisions about the money supply and the increasing (or decreasing) of it. By buying and selling government securities (typically interest rate products such as bonds, bills and notes) they can effect the supply of money. to tighten the money supply, for example, the would sell securities.

The meetings are top secret and are the subject of much speculation by traders across the globe. The market always shows a dramatic increase in volatility on an intra-day basis when the results of this meeting are announced, even when the results are exactly what are expected.

Further information and charts showing historical market activity can be seen here: Fed Day Charts.