Summary

UML & OCL are standards to specify the various aspects of sofware systems (each ones being called a "diagram") :

1. Use Case to describe the functionalities (: what is it for ?)
2. Sequence or object s' communication to represent a particular interaction (: e.g. how to obtain a result ?)
3. Statechart to specify the general behavior of a entity 's type.
4. Classes and packages to define the general software architecture
5. Deployment to define the material one.
6. There are other diagrams that are mostly a composition of the previous one (e.g. classes' diagram inside a class, block part in a sequence diagram linked to a state, conceptual states in activity diagram, etc.).

The language has :

1. A syntax to be known (and to be understood by the others) and
2. A semantic - "objects exchanging messages" as a paradigm to model software.