Although the stock market is the first place in which many people think to invest, the U.S. Treasury bond markets arguably have the greatest impact on the economy and are watched the world over. Unfortunately, just because they are influential, doesn’t make them any easier to understand, and they can be downright bewildering to the uninitiated.

At the most basic level, a bond is a loan. Just as people obtain a loan from the bank, governments and companies borrow money from citizens in the form of bonds. A bond really is nothing more than a loan issued by you, the investor, to the government or company, the issuer.

For the privilege of using your money, the bond issuer pays something extra in the form of interest payments that are made at a predetermined rate and schedule. The interest rate often is referred to as the coupon, and the date on which the issuer must repay the amount borrowed, or face value, is called the maturity date.

One wrinkle in the equation, though, is that not all debt is created equal with some issuers being more likely to default on their obligation. As such, credit rating agencies evaluate companies and governments to give them a grade on how likely they are to repay the debt (see "Good, better, best").

Benji Baily and Delmar King, fixed income investment managers at Everence Financial, say ratings generally can be classified as investment grade or junk. "Anything that’s considered to be an investment grade, you would have a fairly high probability that you’re going to get your money back at maturity," King says. "Of course, the lower you go down the credit spectrum, the more risk there is of default and the possibility that you could have losses. Therefore, the lower the security grade you have, the more yield compensation you should have for taking that default risk."

So, if you purchased a 30-year U.S. Treasury bond (currently AA+ from S&P and AAA from Moody’s and Fitch) for $100,000 with a coupon rate of 6%, then you could expect to receive $6,000 a year for the duration of the bond and then receive the face value of $100,000 back. At least, that’s how a bond would work if you held it to maturity.

Rather than hold a bond to maturity, they also can be traded. But, as a bond is traded, interest rates can change, so the overall value of the bond can change. "If you bought a bond that has a 10% coupon and the rest of the market is fine with owning a 1% coupon, then someone is going to love to have that 10% coupon until maturity," Baily says. "Conversely, if you have a 1% bond and everyone else is expecting that the market in general will be at 10%, then you’re going to need to pay someone a lot of money to take that 1% bond instead of buying a new 10% bond."

Because coupon rates generally are fixed, to adjust for future expectations the price of the bond or note has to move up or down. If yields, the interest or dividends received on a security, go up, the price will fall to accommodate that higher yield; if yields go down, then price has to go up.