The UML is applicable to object-oriented problem solving. A model is an abstraction of the underlying problem. The domain is the actual world from which the problem comes. Models consist of objects that interact by sending each other messages . The values of an object's attributes determine its state .
Objects are instances of classes.
Use case diagrams Use case diagrams describe what a system does from the standpoint of an external observer. The emphasis is on what a system does rather than how. Use case diagrams are closely connected to scenarios. A scenario is an example of what happens when someone interacts with the system. Here is a scenario for a medical clinic.