UML is the acronym for Unified Modeling Language. It is a set of diagrams and supporting text used to represent how an automated system should behave. It is based on the merging of the work of three different OO methodologists: Grady Booch, Jim Rumbaugh and Ivar Jacobsen. Each of them had a particular way of representing different aspects of a problem space to capture how the various objects and components should interact with each other and with external forces and objects. They each authored books, and then all went to work together at Rational Software in the 1990's. It caused quite a stir at the time.
If you are interested in getting more details on the various diagrams and how they fit together, you should visit the Wikipedia article about Rational Software: http://en.wikipedia.org/wiki/Rational_Software It will give you a much better introduction to the history of the Rational Unified Process. I wouldn't bother going to the Rational Software website. The company was acquired by IBM a while ago and their site is notoriously difficult to navigate.