Hi Michael,
There is no actual grade. Typically this is subjective as some will you an A-F scale, while others A-D and so on. I've read somewhere an easy way to remember is A: you would live there, B: you could live there, C: you don't want to live there, D: you won't live there.
On an A-D Scale:
Typically, A class neighbourhoods are those in the city centres, and the better parts of the boroughs; higher incomes, better schools, close proximity to transit. Although exceptions do exist.
B Class are typically borough neighbourhoods, where incomes are lower but the community demographic is still full of professionals; accountants, nurses etc.
C Class can also be in the borough neighbourhoods, but the lower income parts where there are apartments, subsidized housing etc.
D Class are the worst neighbourhoods; abandoned homes, lots of subsidized housing etc.
The same scale is used to base a property, for example an A class property in a B class neighbourhood. Typically good investment properties are C class properties (need work) in B class neighbourhoods.