Corporate Bonds trade in decentralized, dealer-based, over-the-counter markets. In over-the-counter trading dealers act as intermediaries between buyers and sellers. Corporate bonds are sometimes listed on exchanges (these are called "listed" bonds) and ECNs. However, vast majority of trading volume happens over-the-counter.
By far the largest market for corporate bonds is in corporate bonds denominated in US Dollars. US Dollar corporate bond market is the oldest, largest, and most developed. As the term corporate bond is not well defined, the size of the market varies according to who is doing the counting, but it is in the $5 to $6 trillion range.
The second largest market is in Euro denominated corporate bonds. Other markets tend to be small by comparison and are usually not well developed, with low trading volumes. Many corporations from other countries issue in either US Dollars or Euros. Foreign corporates issuing bonds in the US Dollar market are called Yankees and their bonds are Yankee bonds.
Investment Grade v High Yield
Corporate bonds are divided into two main categories High Grade (also called Investment Grade) and High Yield (also called Non-Investment Grade, Speculative Grade, or Junk Bonds) according to their credit rating. Bonds rated AAA, AA, A, and BBB are High Grade, while bonds rated BB and below are High Yield. This is a significant distinction as High Grade and High Yield bonds are traded by different trading desks and held by different investors. For example many pension funds and insurance companies are prohibited from holding more than a token amount of High Yield bonds (by internal rules or government regulation). The distinction between High Grade and High Yield is also common to most corporate bond markets.
The most common derivative on corporate bonds are called credit default swap (CDS) which are contracts between two parties that provide a synthetic exposure with similar risks to owning the bond. The bond that the CDS is based on is called the Reference Entity and the difference between the credit spread of the bond and the spread of the CDS is called the Bond-CDS basis.