Yeah, the wiki page (https://wiki.openstreetmap.org/wiki/Land_use) is probably the best explainer, but landuse=residential is basically "this area is primarily a residential area"

> Should every parcel on the map have a =landuse tag? Should they be separated by neighborhood? 

There definitely shouldn't be a separate landuse per parcel, some people try to do one landuse area for each neighborhood, others go based off aerial imagery.

> that residential area includes parts of two different neighborhoods and a large commercial area. It also excludes the nearby non-profit low income housing and seems to be segregating by race.

I highly doubt the segregation was intentional, that would be a strange thing to intentionally do. The best thing to do in that case is to modify the landuse to exclude the commercial area and include the nearby low income area.