Use cases are a technique for organizing and eliciting requirements. They were originally popularized by Ivar Jacobson in the late 80's and early 90's.
People commonly ask questions about use cases, most of which can be answered by Alistair Cockburn's book, which is by far the best book on use cases. You should read this book before using or asking questions about use cases.
Use cases appear in the UML in the form of use case diagrams, but these diagrams are of little value - the key value of use cases lies in the text which is not standardized in UML. So when you do use cases put your energy into the text.
Use case are at their best when they are short and readable. You should not be spending weeks, let alone months, generating use case documents before you begin development.
Also see UseCasesAndStories