Social grade is a classification system based on occupation. It was developed for use on the NRS (National Readership Survey) and for over 50 years NRS has been the research industry's source of social grade data.
(Table from NRS website)
A - Upper Middle Class
B - Middle Class
C1 - Lower Middle Class
C2 - Skilled Working Class
D - Working Class
E - Those At The Lowest Level Of Subsistence
Upper Middle Class - The social group constituted by higher-status members of the middle class. The upper middle class is defined as consisting mostly of white-collar professionals who have above-average personal incomes and advanced educational degrees.
Middle Class - The social group in the middle of a societal hierarchy. The middle class is the broad group of people in contemporary society who fall socio-economically between the working class and upper class. College-eductaed workers.
Lower Middle Class - Semi-professionals and craftsmen with a roughly average standard of living. Most have some college-eductaion.
Skilled Working Class - A section of society dependant on physical labour and specific trades such as plumbers and electricians.
Working Class - Social groups employed in lower tier jobs often extending into unemployment or otherwise possessing below average incomes.
Those At The Lowest Level Of Subsistence - Casual tr lowest grade workers. People who depend on the welfare state for their income.
The household is classified according to the CIE’s occupation, but social grade data are also available for the respondent themselves (if they are not the CIE).
Income is not part of the social grade classification. However there is a strong correlation between income and social grade as the following chart shows.
(Table from NRS website)
Demographics and the social grade table are used to establish how to create a product to appeal to a certain target audience. Looking at certain groups of peoples income and lifestyle can help you design a product suited to them and help to predict how well it could sell.