What are the 3 classifications of debt investments?

Could you please explain to me the three main categories of debt investments? I'm particularly interested in understanding the distinctions between them and how they differ in terms of risk, return, and other key factors. As a potential investor, I want to ensure that I make informed decisions about my portfolio allocation. Additionally, I'm curious about the pros and cons associated with each type of debt investment, and how they fit into a broader investment strategy. Could you elaborate on these points?