Debentures are capital raising strategy. This is non-collateralised debt rised by the borrower on the creditworthiness of the borrower. This has a lower risk and thus lower appreciation. They are illiquid and not readily traded in the secondary market. It is used by both corporations and governments.
T-Bills and Notes and T-Bonds are types of debentures. They are same as ordinary bonds except that they are not collateralised with any asset but only on trust and credit-worthiness of the borrower.
Some debentures are convertible i.e. they come with an embedded option where the lender can convert the debentures into other securities (ex. stocks) after a while. These kind of debentures are low interest yielding debentures.