A case study is a research-based method used to investigate an individual, group, or event to explore what caused something, and to find the underlying principles that drive that cause.
Case studies involve an in-depth, long term examination of a single event - a case.
There is usually a systematic way of looking at events, collecting data, analyzing information, and reporting the results. Case studies are great for generating and testing hypotheses.
An example would be a consulting company that has a website and is trying to sell their services to another company. The consulting company can perform a "case study" on one of their previous projects to show how their contributions helped to achieve some defined goal of their client.
In keeping with our GM example, here is a case study that can be used to understand the electric vehicle marketplace.