The G Fund - The G Fund invests in short-term U.S. treasuries.  Principal is guaranteed.

The F Fund - The F Fund is invested in the Barclays U.S. Debt Index Fund, bond index fund which holds a representative sample of the bonds in the Lehman Brothers U.S. Aggregate (LBA) index.  Economic indicators and money supply affect interest rates.  Interest rates influence the movement of this fund.  When yields move lower, bonds go up in price and the F fund goes up in value.  When yields move higher, bonds go down in price and the F fund goes down in value.  

The S Fund - Small cap is a bit of a misnomer for this fund.  It is actually all of the U.S. stocks that are not included in the S&P 500.  There are some fairly large companies in this fund.  It is really a compilation of mid-cap stocks and small cap stocks.  It tracks the Barclays Extended Market Index Fund, which tracks the Wilshire 4500 index .  By investing in both the C and S funds, you are basically investing in every tradable U.S. company.

The I Fund - The international fund follows the *EAFE index.  This stands for Europe, Australia and Far East.  The EAFE countries are Australia, Austria, Belgium, Denmark, Finland, France, Germany, Hong Kong, Ireland, Italy Japan, Netherlands, New Zealand, Norway, Portugal, Singapore, Spain, Sweden, Switzerland and United Kingdom.  If you surf through any of the financial networks on TV at night, you will see quotes on these exchanges and those results affect the next day's I fund price.  The I Fund also carries the risk of foreign currency fluctuations.  The stock prices of the companies in the EAFE index are expressed in the currency of each respective country and then converted to U.S. dollars to determine the value of the EAFE index.  Thus, the value of the EAFE index will rise as the value of the U.S. dollar falls — and fall as the value of the U.S. dollar rises — relative to the currencies of countries with companies that are represented in the EAFE index.