What are the differences of class A, B, and C office buildings?

Biggest difference is Location. A new downtown Boston Building would be considered A, where the same building in a suburban market might be a B

A, B, and C have to do with how nice your building is compared to other buildings in that market.
A building that offers better than average ammenities and services is class A, average amenities is class B, and lower than average is class C. A class B building in one location could be a class A in another.

The different classes are subjectively assigned based on a combination of location, building/amenities quality and condition, and tenant strength. A building can start out as Class A and gradually decline to Class C as it depreciates and competes with newer product. A building can also receive extensive renovation and jump up a class. A building with Microsoft as the main tenant would be Class A, whereas a building primarily occupied by Bill's Bail Bonds may be Class C.