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.