UML basics: An introduction to the Unified Modeling Language
from The Rational Edge: This introduction to the Unified Modeling Language, or UML, provides an overview of the most important diagrams used in the visual modeling of computing programs. The article is ideal for those who have little knowledge of UML concepts, including managers as well as novice practitioners.
Although UML as whole is IMHO dead, some of the diagrams are still quite usefull.