A debenture is a fixed income financial instrument which pays regular interest and has a set maturity date. In most respects it is like a bond, and the term bond is often used to mean both bond and debentures. The main difference between the two is that a bond is secured against a specific asset of a company or government, while a debenture is secured only against the general credit of the issuer. (In North America, anyhow.) So if the issuer were to go under, a debenture would be a less protected investment than a bond.

In practice, this is only really a concern with corporate debentures - most governments are unlikely to go under (and if they do, you have more serious problems than worrying about your investments), and even if they do find themselves short on cash, they have the power to levy taxes or print more money to pay them off. Of course, the government would have to be really desperate to do that, since it would depress their economy and send their currency's inflation sky-high, making foreign investors very unwilling to invest in them, but that the power exists means that most government debentures are considered fairly secure.