In corporate finance, a debenture is a medium- to long-term debt instrument used by large companies to borrow money, at a fixed rate of interest. The legal term "debenture" originally referred to a document that either creates a debt or acknowledges it, but in some countries the term is now used interchangeably with bond, loan stock or note. A debenture is thus like a certificate of loan or a loan bond evidencing the fact that the company is liable to pay a specified amount with interest and although the money raised by the debentures becomes a part of the company's capital structure, it does not become share capital.
All the classes of shares above are rewarded with dividends. that is, if you subscribe to any of the shares, the company would pay you dividends as returns on investment, while debenture is an certificate that acknowledges a debt owed by one person to another.