Application conceptualization and development becomes more efficient when Unified Modeling Language diagrams are employed. Development teams are able to better envision overall application structure and functionality devoid of programming language specifics enhancing portability and team communication.
This introductory tutorial is broken into nice learning chunks with good illustrations. http://www.cragsystems.co.uk/uml_tutorial_download.htm
This paper provides a nice overview of uml diagrams and is still a very useful learning tool even though it references an older version of UML. http://tinyurl.com/uml-intro
Here's are two nice discussions of class diagrams. http://tinyurl.com/umlclassdiagram http://www.agilemodeling.com/artifacts/classDiagram.htm
Here is a nice free UML Modeling Tool. http://argouml.tigris.org/
Here is an experimental online uml diagramming tool where you can create quick diagrams to save your ideas or post them to the web. http://yuml.me/diagram/scruffy/class/draw
Here is a fine book on UML by two of the UML's original authors. http://tinyurl.com/uml-textbook