Debentures are always different from stocks and bonds, but basically, all these three are different forms of investment. The difference is mainly because of the varying risk factors and returns reflected by the higher risks and corresponding higher returns of the debentures.
When somebody buys a stock, he/she becomes one of the owners of the company and faces 'floating' fortunes, related with the company's financial graph. But debentures are more secured investment as payments with high interest rates are guaranteed. Government is bound to pay interest on the borrowed money and once the debenture matures, all the borrowed money is returned. So, this is understandable that, it is actually the interest which one gains from the government debentures.
In both of these investment forms, interest and value is guaranteed, but in case of liquidation, bond holders receive the payment first. Debenture holders have no collateral, which they can claim from the company, in case bankruptcy takes place and to compensate such losses high interest rates are paid to them. The bondholders enjoy the collateral.