UML Class Diagrams
The Unified Modeling Language is a means of documenting the responsibilities of classes and their relationships.
Use Case Modeling
A Use Case is a specific instance of a system in use. It is a scenario of how the system will be used by a specific user or class of users. The external specification of the system can be given as a set of use cases. The use cases are later used to verify the system.
Classes are built to model the use cases.