Use Case Diagrams

Use case diagrams are a form of visual representation of a specific system, it is similar to UML diagrams, except that use case diagrams are more general and can be applied to any kind of system, process or anything like those. Use case diagrams are used to ilustrate the various processes, steps, and relationships between different elements of a system. An example of one is the following:

 

 Elements of a Use Case Diagram:

a) Actors: An element (usually a person), that must interact with the system and exerts change.
b) Requirements: Enclosed in Ovals, these represent the specific actions that must be done. It is important to note that these actions follow a specific syntax, that of verb + noun, for example, in the above diagram, one action that can be performed is buy painting.
c) Use Case Relations: These are the lines that connect the various idead together.

Other Use Case Diagram Relationships:

a) Inheritance: You use this whenever you want to ilustrate a scenario in which a specific class acts slightly different than the general idea. For example, creating a publisher blog is a specific form of creating a blog.
b) Extends: Whenever you want to create a similar scenario to the main general idea, but that has the capacity to change if the situation warrants it. For example, normally a program would execute commands A,B, C, however if something goes wrong it might function as A, B,D. 
c) Include: Where a use case at some point reuses all of the steps of another use case.