The UML stands for Unified Modelling language and it is one of the great tools for Object-oriented design. It allows you to create several diagrams like class diagram, sequence diagram, object diagram, etc, which helps you to understand your system better. The UML also helps you to convey your thoughts and design to peers and team members before implementation. Since a picture is worth a thousand words, those interactions often help you to find shortcomings and loopholes in your software design very early. That's actually the reason I use UML in our projects. Btw, I didn't know about UML or UML diagrams when I first started Java development.