If you want to sell your bond before the maturity date, you take the risk to receive less money than the capital invested. The value of a bond increases if the interest rates decrease and decreases if the interest rates increase. During the issue period, the rate of your bond reflects current market conditions. If, later, newly issued debt securities of the same type offer coupons with a higher rate, your bond will be less attractive and its value will get lower.
There are several kinds of bonds, including convertible bonds (the holder can convert the bond into a specified number of shares of common stock in the issuing company or cash of equal value), puttable bonds (the holder of a puttable bond has the right, but not the obligation, to demand early repayment of the principal on one or more specified dates called windows periods) and zero-coupon bonds (when a zero-coupon bond matures, the investor receives one lump sum equal to the initial investment plus the imputed interest).