As I understand a debenture is an offer to the general public from a business as a means of finance. They issue debentures to a number of individuals (can be up to thousands) which is basically terms of a 'loan', where an individual offers an amount of money to invest into the business. The debenture thats offered has a "return" on this investment (eg: 6%) that the indiviudal who loaned the money will recieve based on the terms outlined in the prospectus (eg: per annum, quarterly, or even a lump payout with interest on maturity in a few years).
Once you get the idea, its pretty basic .. but yes, initially I had problems discerning between a debenture and bond >< If youre still having problems grasping it, ill copy and paste the textbook definition and walk through it