A case study, in a business context is a report of an organization's implementation of something, such as a practice, a product, a system or a service.
The case study can be thought of as a real-world test of how the implementation works, and how well it works. If documentation is sufficiently comprehensive, a case study should yield valuable information about the costs and benefits, both financial and otherwise. The organization itself and external parties can use case studies to gain more information about the specific implementation that can also help guide decisions about similar projects.
An organization might deploy, for example, a new business intelligence (BI) platform. A case study of its implementation would involve an exploration of each stage of the implementation, lessons learned along the way and the ultimate effects.
A case study is often compiled by an external party, perhaps for publication. Less formally, a case study may just be a content item, such as a brief article, discussing an organization's business implementation of something or a decision of some sort and one or more significant results.