How are credit ratings like AAA and BBB classified?

For long-term credit ratings, AAA is the highest, followed in descending order by AA, A, BBB, BB, B, CCC, CC, C and D, with + and - used for finer gradations within the same grade.

Generally, BBB- and above is classified as investment grade and BB+ and below as speculative grade. D indicates default, and short-term ratings are expressed separately as A1, A2, A3, B and so on.