Case study is a common expression in qualitative research and the use varies enormously amongst time and researchers, and, sometimes, its meaning is just taken for granted.
After some time also using the expression vaguely, I found the chapter So, what are case studies? in the book What's wrong with ethnography?, by Martyn Hammersley. The author has published many books about research in education, therefore, his ideas must be seen as part of a whole set of ideas and definitions.
What I like about him is not only the quality and clarity of his writings, but also his opposition to the traditional division between qualitative and quantitative research and his straight forward definition of terms that are used quite often (and losely) in educational research, such as ethnography and case study.
In this post, I will present briefly his definition of case study by presenting the following scheme he suggests in order to describe a research design:
From the scheme above, it is clear that the author uses a "much narrower definition of case study than conventional" (p. 185). The author considers case study as one of the options for selection of cases together with experiment and survey. Then, to characterize the strengths and weaknesses of a case study, the author compares it with the other two options.
Mainly, comparing case studies to surveys, the author argues that there is a trade off between number of cases (then, representativeness of the sample) and amount of detail (then, degree of likely accuracy). When comparing case studies to experiments, the authors argues that there is a trade off between degree of control (then, possibility of coming to sound conclusions about causal relationship) and degree of reactivity (then, artificiality of the results).
I imagine that his ideas may sound simplistic for some people involved in educational research, but they are very coherent specially when one takes into account all of Hammersley's ideas about research design.
Rererence: Hammersley, M. (1992). What is wrong with ethnography? Mixing Methods: Qualitative and Quantitative Research. Routledge.