By definition a high-yield bonds is a high paying bond with a lower credit rating than investment-grade corporate bonds. It also has a lower credit rating than Treasury bonds and municipal bonds. The upside of this is that because of the higher risk of default, these bonds pay a higher yield than investment grade bonds.
Based on the two main credit rating agencies, high-yield bonds carry a rating of ‘BBB’ or lower from S&P, and ‘Baa’ or lower from Moody’s. Bonds with ratings above these levels are considered investment grade. Credit ratings can be as low as ‘D’ (currently in default), and most bonds with ‘C’ ratings or lower carry a high risk of default; to compensate for this risk, yields will typically be very high.