Visualizing systems displays a graphical of the system to allow for easier understanding.

In terms of specifying systems, it is used to say that the system is and what is to be done in order for the system to function when completed.

In terms of Constructing systems, it is used as a path/guide to following when constructing the system.

Documenting systems, the UML is used to record certain vital information about the system under development. 

In terms of a system-intensive system, a method is used to depict what the system will become or is to become.

What the UML is not?

Contrary to popular belief the UML is not a programming language like C++ or Java, but it is used to support the development of a system using one of those languages or a language of choice.