按特點來分有secured 和unsecured 兩種。其中unsecured bond 叫做debenture。
補充資料：
這兩個都是公司為提高現金流而籌資的債務工具。
異同點：
（1）Bond 債券
IOU， between an issuer and investor， when the investor loans its money to an institution， a bond exercise a written promise that borrower makes to repay the loan on a specific maturity date.
Bonds are fix income security， normally come with periodic interest payments as well. They are considered to be a safe investment， bonds from governments or highly rated corporations have lower default risk.
Bond最主要的特征是IOU，這可不是我愛妳的意思，而是I owe you，我欠妳的意思。當然是發債人欠投資人了，債券作為書面承諾，保證借款人在特定的期限屆滿日向貸款人還錢。
Bond屬於固定收益債券，通常會定期支付利息，被視為相對安全的投資方式，政府或評級高的公司債券，違約風險更低。
（2）Debenture 無抵押債券/信用債券
Debenture is a type of debt instrument that is not secured by physical assets or collateral. Debentures are backed only by the general creditworthiness and reputation of the issuer. Like other types of bonds， debentures are documented in an indenture. Debentures have extra risk and higher interest rate.
Debentures have no collateral. Bond buyers generally purchase debentures based on the belief that the bond issuer is unlikely to default on the repayment. An example of a government debenture would be any government-issued Treasury bond (T-bond) or Treasury bill (T-bill). T-bonds and T-bills are generally considered risk-free because governments， at worst， can print more money or raise taxes to pay these types of debts.
Debenture是壹種無實物資產抵押或擔保的債務工具，僅靠發債人的信用和商譽強撐。和其他債券類型壹樣，Debenture被視為壹種契約。Debenture也是高風險高收益的。
此種債券的購買者通常認為該公司或政府不太可能還不上錢，上面提到的三種美國國債就屬於Debenture，它們壹般被視為無風險債券。最不濟，大不了政府多印點錢，或者多收點稅就捎帶腳的把錢給還了嘛。
When insolvency happens， Bond holders own the title of secured assets; Debenture holders have to try their luck in bankruptcy court， they have to stand in line behind other bond holders.
別看無抵押債券持有人拿利息的時候笑嘻嘻，真到公司面臨破產清算了的時候，不壹樣心裏MMP？人家有抵押債券持有者直接就把抵押物賣了按比例分掉，妳不只能可憐巴巴看著公司走完清償法律流程，等排在前面的高級債權人和其他債券持有者分完蛋糕之後，舔個渣嘛？可能壹般連渣都不剩了。
A company may add certain features to make a debenture more attractive， e.g Convertible Bond; Create a sinking fund. Both features reduce debt holder risk， resulting in smaller coupon payments.
這風險聽起來太大了，有沒有把大家都嚇跑？其實不至於，而且公司為了提高無抵押債券對投資者的吸引力，還提供了其他選擇，比如可轉換債券，償債基金。這兩種模式降低了風險，自然支付的收益也就減少了。