When an investor buys a bond in between coupon payments, he is supposed to compensate the seller with the coupon interest earned on the bond from the last coupon payment date to the settlement date. This amount of interest is called accrued interest, so the buyer pays the seller the agreed price plus the accrued interest. This is known as full price. The price of the bond without the accrued interest is known as clean price.
A bond in which the buyer must pay the seller accrued interest is said to be trading cum-coupon. If the buyer forgoes the next coupon payment, the bond is said to be trading ex-coupon. In the government bond market in India, and in most other bond markets around the world, the buyer has to pay accrued interest to the seller.
Suppose a bond pays interest semi-annually on July 1 and January 1. If a person sells the bond on May 1, he gets no interest for the four months from January 1 to April 30 for which he held the bond, while the buyer would get six months interest on July 1 though he held it only for two months (May 1 to June 30). The interest for the period from the last coupon due date to the date of the sale is known as accrued interest. In the above illustration, if the bond has a face value of Rs.100 and carries a coupon of 12%, then the accrued interest would amount to Rs.100 x 12/100 x 4/12 = Rs.4.
It is often a convention in the bond markets that the buyer pays the accrued interest to the seller in addition to the price. In other words, the actual cash price paid is equal to the quoted price plus the accrued interest. In India, this practice is prevalent in the government bonds market, but not in the corporate bonds market. In the above illustration, if the quoted price is Rs.98 then under this convention, the actual cash price would be Rs.98 + 4 = Rs.102.