What are the differences between a use-case diagram and use case?

Use Case diagrams are behavior diagrams of UML used to describe a set of actions (Use Cases) that some system or systems (Subject) should or can perform in collaboration with one or more external users of the system (Actors).

Each Use Case describes a unit of complete and useful functionality, some observable and valuable result that the subject provides to its users (actors)or other stakeholders of the system.

Example use cases for bank ATM are: Withdraw Cash, Make Deposit.

Use case diagram for ATM could show users of ATM, some or all of use cases, boundary of the system. See example in provided link.