Use Case Modeling

Introduction to  UML 2 Use Case Diagrams

http://www.agilemodeling.com/artifacts/useCaseDiagram.htm


Use cases. A use case describes a sequence of actions that provide something of measurable value to an actor and is drawn as a horizontal ellipse. 

Actors. An actor is a person, organization, or external system that plays a role in one or more interactions with your system. Actors are drawn as stick figures. 

Associations.  Associations between actors and use cases are indicated in use case diagrams by solid lines. An association exists whenever an actor is involved with an interaction described by a use case.  Associations are modeled as lines connecting use cases and actors to one another, with an optional arrowhead on one end of the line. The arrowhead is often used to indicating the direction of the initial invocation of the relationship or to indicate the primary actor within the use case.  The arrowheads are typically confused with data flow and as a result I avoid their use. 

System boundary boxes (optional). You can draw a rectangle around the use cases, called the system boundary box, to indicates the scope of your system.  Anything within the box represents functionality that is in scope and anything outside the box is not.  System boundary boxes are rarely used, although on occasion I have used them to identify which use cases will be delivered in each major release of a system.

Packages (optional).  Packages are UML constructs that enable you to organize model elements (such as use cases) into groups. Packages are depicted as file folders and can be used on any of the UML diagrams, including both use case diagrams and class diagrams. I use packages only when my diagrams become unwieldy, which generally implies they cannot be printed on a single page, to organize a large diagram into smaller ones.

Writing effective Use Case Examples
http://www.gatherspace.com/static/use_case_example.html#1

UML Use Case Diagrams: Tips
http://www.andrew.cmu.edu/course/90-754/umlucdfaq.html