Introductory article on UML: Practical UML™ - A Hands-On Introduction for Developers

Practical UML™: A Hands-On Introduction for Developers - by Randy Miller provides a quick but clear introduction to the Unified Modeling Language. Here you can grasp the basic concepts behind the 9 kinds of modeling diagrams (use case, class, object, sequence, collaboration, statechart, activity, component and deployment diagrams).