UML (Unified Modelling Language) is a standard object oriented method of documenting the design of a system. It provides language and diagram standards that allow you to easily document the key points of how your system is required to function and can be as high level or as detailed as is required by the different stakeholders involved with the system.
All of the parts of the UML are optional and only those sections that are required for a given task need to be included in your documentation.
The main part of the UML standard is concerned with the diagramming of the model. The following diagraming standards are provided to cover both static and behavioural modelling.
This article written by Stephen Chapman, Felgall Pty Ltd.