Interest Rate Risk: When interest rates rise, bond prices fall. So if the fund manager has his portfolio stacked with lower interest rate paper, the prices of his holdings will fall resulting in a lower NAV. On the other hand, if interest rates fall then the price of his holdings rise and so does his NAV.
The longer a bond's maturity, the greater the interest rates risk. A bond fund with a longer average maturity will see its net asset value (NAV) react more dramatically to changes in interest rates as the prices of the underlying bonds in the portfolio increase or decline.
Credit Risk: Bonds carry the risk of default, meaning that the issuer is unable to make further interest or principal payments. They are rated by individual credit rating agencies to help describe the credit worthiness of the issuer. Higher the credit rating, lower the risk and lower the returns. Lower the credit rating, higher the risk and higher the return.
Liquidity Risk: If the credit rating gets downgraded or the current interest rates are much higher than the coupon rate, then the bond would face liquidity issues because finding a buyer would no longer be easy. Liquidity risk describes the danger when one has to sell a bond in the secondary market but is unable to find a buyer.