A use case describes how a primary actor interacts with a system. It also shows any interactions between use cases or secondary actors (such as timers or external system triggers). 

Use cases are illustrated in a use case diagram that consists of the following notational elements: 

• Use cases: The functionality of a solution.
• Actors: The users that interact with the solution in different ways.
• Relationships: Show how the actors interact with the different use cases shown in the diagram. 

There are two types of relationships between use cases—namely the following:

• Include: This relationship shows that the use case will always include the functionality of another as part of its execution.
• Extend: This relationship shows that a use case will extend to use the other use case's functionality as part of its own when certain conditions are met.

Each use case that is identified in a use case diagram is supported by a detailed use case description. Each use case description includes information including the title of the use case, any conditions that the use case has to meet before it can be activated or used, the sequence of activities that are performed during its execution, and any exceptions that may exist.

The use case's description also includes information about which actors use it, what the trigger for it to execute is, and the overall purpose of its existence.