Bonds are debt instruments, or agreements to repay capital plus interest on set dates. They are issued by companies or governments (bond issuers) to raise money. As an investor, you are, in effect, lending money to a company or government.
Bonds are both listed and unlisted. You can buy bonds from the issuers during their Initial Public Offering (IPO). An example of this is the Hong Kong government's ibonds, which are Hong Kong dollar retail inflation-indexed bonds.
If bonds are listed on the stock exchange, you can trade them like stocks. If unlisted, you can trade them on the secondary market, through banks and brokers.
Some investors have bonds to earn a steady income. Others try to profit by trading them. Like any investment, there are risks for investing in bonds, and the key one is credit risk. This means you depend on the bond issuer to pay interest on time, and the principal when the bond matures. It is important to read the bond offering's prospectus. This will give more details of the bond's characteristics and associated risks.