The main difference is the credit rating of the bond seller. For example, the UK and US governments have excellent credit ratings so they are safe but offer really low yields. Companies also issue bonds and they are obviously riskier than governments but offer better yields. Also short term bonds are less risky, because you get your money (principal) back quicker and lower probability of interest rate increase so they offer lower yields than long term bonds. If you go to the bond fund manager website you can have a look at the distribution of the bonds in the funds and the credit ratings. Credit ratings go from AAA, AA, A, BBB, etc all the way to C and D (default), bonds over BBB are considered high grade or investment grade and anything below are high yield or “junk” bonds, which I wouldn’t recommend.