what are meant by social classes when talking about market segments?

[just someone]

[wendy c]

Segmentation is the term used to describe the process of assigning people, places, or things into homogeneous groupings.

You can segment people by a variety of known factors including:

Age
Income
Sex
Race
Political leanings
Purchase behaviors (cars driven, credit cards used, etc.)
ETC.

A social class is generally thought of as a group of people who have varying degrees of income, education, and occupation which lead them to come to similar conclusions in many cases.

People refer to "the upper class", "the middle class", "the working class" in order to better understand their opinions, choices, and circumstances within an easily identifiable context.

There is a really good explanation of social class on Wikipedia...referenced below.