A use case diagram is a behavioral diagram that is very commonly used in software development. The main purpose of this diagram is to illustrate the functional usage of a piece of software. It holds the use cases of a system and can be used to provide a high-level view of the functionality or even a very specific low-level module of a software. Normally for a system, there are multiple use case diagrams that focus on the different levels of the system. Use case diagrams shouldn't be used to display the implementation details of a system; they were developed to show only the functional requirements of a system. Use case diagrams are very good diagrams for business people to convey what they need from a system.

There are four main parts of a use case diagram, as shown in the following list:

• The actor
• The use case
• The communication link
• The system boundaries