There are essentially two types of use cases; essential use cases and system use cases.
Essential use cases focus on the 'what' not 'how' by trying to capture the intentions of a user in a technology independent manner whereas system use cases include high-level implementation decisions.
The essential UseCases presented are documented using modified methods and templates described by S.W.Ambler and Astrogrid.
Also, visit the The Use Case Zone for more details on use cases.
-- AnilWipat - 04 Jan 2003