papges at: http://www.omg.org/technology/UML and http://www.omg.org/technology/documents/formal/.

What is the UML?

The UNIFIED MODELING LANGUAGE (UML) is a graphical language for visualizing, specifying, constructing, and documenting the artifacts of a system ¨C intensive system (Booch, 1999).  The modeling language allows for a visual expression of the software system being developed.  This is achieved by using various UML notations and diagrams.  To serve its purpose, a modeling language has to allow concise expression of the essential aspects of the software system being developed while omitting irrelevant detail (Selic, Ramackers & Kobryn, 2002).  More than likely the system developed using UML will use one of the popular object-oriented programming languages such as SmallTalk, C++ and Java.

As a modeling language, it gives a graphical description of the system using whatever diagrams that are needed to do so.  In terms of a language, it allows for a common method of communication between stakeholders.  This basically is to gather information about the system and how it communicates with each other.