Physically, UML is a set of specifications from the OMG. UML 2.0 is distributed as four specifications: the Diagram Interchange Specification , the UML Infrastructure, the UML Superstructure, and the Object Constraint Language (OCL). All of these specifications are available from the OMG web site, http://www.omg.org.
The Diagram Interchange Specification was written to provide a way to share UML models between different modeling tools. Previous versions of UML defined an XML schema for capturing what elements were used in a UML diagram, but did not capture any information about how a diagram was laid out. To address this, the Diagram Interchange Specification was developed along with a mapping from a new XML schema to a Scalable Vector Graphics (SVG) representation. Typically the Diagram Interchange Specification is used only by tool vendors, though the OMG makes an effort to include "whiteboard tools."
The UML Infrastructure defines the fundamental, low-level, core, bottom-most concepts in UML; the infrastructure is a metamodel that is used to produce the rest of UML. The infrastructure isn't typically used by an end user, but it provides the foundation for the UML Superstructure.
The UML Superstructure is the formal definition of the elements of UML, and it weighs in at over 600 pages. This is the authority on all that is UML, at least as far as the OMG is concerned. The superstructure documentation is typically used by tool vendors and those writing books on UML, though some effort has been made to make it human readable.
The OCL specification defines a simple language for writing constraints and expressions for elements in a model. The OCL is often brought into play when you specify UML for a particular domain and need to restrict the allowable values for a parameter or object. Appendix B is an overview of the OCL.
It is important to realize that while the specification is the definitive source of the formal definition of UML, it is by no means the be-all and end-all of UML. UML is designed to be extended and interpreted depending on the domain, user, and specific application. There is enough wiggle room in the specification to fit a data center through it... this is intentional. For example, there are typically two or more ways to represent a UML concept depending on what looks best in your diagram or what part of a concept you wish to emphasize. You may choose to represent a particular element using an in-house notation; this is perfectly acceptable as far as UML is concerned. However, you must be careful when using nonstandard notation because part of the reason for using UML in the first place is to have a common representation when collaborating with other users.
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