What determines interest rates in the bond market?

Well, it's not a catchy title, but...

There are many factors that determine interest rates in the bond market. There are short term, intermediate term, and long-term interest rates.

For interest rates, one of the greatest influences is the Federal Reserve Board. It determines the discount rate which, in turn, affects the prime rate for banks. Most loans are dependent upon the prime rate. However, because the Federal Reserve Board meets no more than once a month, this doesn't happen every day.

If you've ever tried to lock-in a mortgage rate, you know how quickly rates change. So, what or who is responsible for the daily rate fluctuations?