Demographics provide a way in which the target market can be divided. Variables you can use include "age, family size, family life cycle, gender, income, occupation, education, religion, race, generation, nationality, and social class."*
This section provides selective sources where you can find demographic information about a target market. They may help you determine whether the target is stable, whether it is substantial enough to let you make a profit, and if it is different from other market segments.
Kotler, P. and Keller K.L., Marketing Management, 14th ed., Boston: Prentice Hall, 2012, p. 216.