I found these articles while I was searching for some basic UML stuff on net.
UML Fundamentals -1 briefs basics of UML, Usecase Diagrams and Sequence Diagrams.
UML Fundamentals -2 gives details of Collabration Diagram, Class Diagram and State Transistion Diagram.
UML Fundamentals -3 throws light on Activity Diagram, Component Diagram, Deployment Diagram and finally Advantages and conclusion.
Links:
UML Fundamentals 1: http://www.microsoft.com/india/msdn/articles/274.aspx
UML Fundamentals 2: http://www.microsoft.com/india/msdn/articles/282.aspx
UML Fundamentals 3: http://www.microsoft.com/india/msdn/articles/283.aspx
I hope this will be helpful for the people who want to learn UML or have a brief reference of it.
Give it a look it's worth it !