There's no hard cute definition, they largely mean same thing to same people, look at it this way:
A - white collar execs, well off business owners, median household income in the 100k+, schools at least 8 out of 10. Virtually lowest crime. Generally properties here don't cashflow with 20 down. Education level mostly college (50%+ and 15%+ advanced degrees)
B - blue collars hard working middle class. Household income min 70k, decent schools. Great CF. State average crime level.
C - already lower household income usually 40k/yr, higher crime, mixed bag of high school and college education. renter density in suburban areas generally higher in the 20%+
D - warzone, high crime, sect 8 mostly living on govt programs and supplementing income via not so legit ways (read meth houses, etc). Property values under $30k, average household income is max $35k/year. You gotta know what you're doing or be the "hefe" on the street to collect rents there.
Im sure other people can add various angles to the same definition.