Do the following self-made sentences have any semantic difference or they are conceptually identical:
- We must help the underprivileged classes of our society.
- We must help the underprivileged segments of our society.
Classes are ranked, i.e. class A could be higher or lower by some attribute (economic, etc.) than class B.
Segments are separated, but not necessarily ranked.
As you are already ranking by saying underprivileged, you should say classes.
To give UK examples of @LawrenceC's distinction there is a widely-used commercial segmentation called "ACORN" that is built on 59 different household segments grouping clustered into 17 groups in turn formed into 5 top-level groups.
For example the 3rd main category ("Comfortable Communities") is made up of "Countryside Communities", "Successful Suburbs", "Steady Neighbourhoods", "Comfortable Seniors" and "Starting out".
The "Successful Suburbs" are inturn made up of "Comfortably off families in modern accomodation", "Larger family homes, multi-ethnic areas" and "Semi-professional families, owner-occupied neighbourhoods"
The traditional UK social class occupation/class distinctions used for governmetn and academic analysis were "A-B-C1-C2-D-E" based on the occupation of the head of the household but roughly corresponding to Upper Middle Class, Middle Class, Lower Middle Class, Skilled Working Class, Working Class, Non-working". (This system has now been replaced by a much more complicated and hard to evaluate system)