First and most importantly, I will define what a use case is. A use case is a tactic used in system analysis to identify, clarify, and categorize system requirements. The use case is made up of a set of possible sequences of interactions between systems and users within an environment and related to a particular goal. The use case should contain all system activities that have significance to the users within a given system.

What Use Cases Do

1.    They hold Functional Requirements in an easy to read and tracking format.

2.    They represent the goal of an interaction between an actor and the system.

3.    They are multi-level, one use case can use/extent the functionality of another.

What Use Cases Do Not Do

1.    They don't specify user interface design. They specify the intent, not the action Detail.

2.    They don't specify implementation detail.

Definitions

1.    Actor: An actor is something with behavior, such as a person, computer system, or organization.

2.    Scenario: A scenario is a specific sequence of actions and interactions between actors and the system under discussion; it is also called a use case instance. It is one particular story of using a system, or one path through the use case; for example, the scenario of successfully using an ATM machine to withdraw cash.

Now that I have introduced what a use case is, what they do, what they do not do, and provided the definition of an actor and scenario, let us look at a complete UML diagram of the ATM cash withdrawal transaction. While this diagram is not all inclusive to actions involved with using an ATM, it should provide the foundation as to how to create a use case diagram and how to interpret its meaning. 

Figure 1: Example UML ATM System Usage Use Case

Now that I have covered the definition of a use case, what they do, what they do not do, and provided a sample diagram, we will move into the various sections contained within a use case.