A use case realization provides a construct to organize artifacts which shown how the physical design of a system supports the logical business behavior outlined by a used case.  To show this traceability between the logical and physical design, in the use case diagram each use case is depicted as an oval shown with a solid-line border.  Then for each use case can you may show a use case realization as an oval with a dotted-line border.  The use case and its corresponding use case realization are linked using a realization dependency which is shown in UML as a dashed line with a triangular arrowhead at the end corresponding to the realized element (the use case). 

Each use case realization will define the physical design in terms of classes and collaborating objects which support the use case.  Therefore, each use case realization typically is made up of a class diagram and a number of interaction diagrams, most commonly sequence diagrams, showing the collaboration or interaction between physical objects.

Use case realizations allow the analyst to clearly separate the concerns of the logical and physical design.  Since a logical design (the business behavior and requirements) can be implemented or realized via a number of different physical implementations, if a physical design changes the logical use case can remain unaffected.  Also, a use case realization is an excellent form of requirements traceability from the logical business requirements down to the physical implementation of the solution.