Answer:
A case study is a research method involving a single or small group of individuals.
A case study, from a Project Management perspective, is mainly telling the story of a project (it can be just part of a project or a phase of a project, or even the whole project), and how it went (whether it succeeded, failed, etc...)
If u don't mind Mr./Ms. PureKnowledge can you please tell it in a way that a 10-11 years kids understand.