What is difference between use case and use case diagram?

A use case outlines a specific goal and how to achieve that goal.

A use case diagram is a graphical representation of system users(actors), use cases, and the system (subject) to which this set of use cases applies.

In more detail:

A use case outlines an interaction between an actor and a system (or a system and a system). For example, the interaction (or dialog) of someone accessing an ATM would look as follows:

Use case name: Authenticate user

1. ATM displays welcome message to Customer

2. ATM asks user to insert card

3. Customer inserts card into ATM

4. ATM asks user to enter their PIN

5. Customer enters their PIN

6. ATM authenticates the customer using the information from the card and the PIN

A use case is functional and outlines the behavior of a set of interactions that form a coherent dialog in the system, i.e. Authenticate user. This interaction is separate and distinct from the use case Withdraw money, which also involves a series of interactions.

A use case diagram shows the relationship between the actors and the use cases. For example, on the Use Case diagram for an ATM you would have the following relationships:

• Authenticate user (dialog fragment above)
• Withdraw money
• Deposit money
• Transfer money
• Get balance

• Provision ATM with envelopes and money
• Remove deposit envelopes

A set of use cases can be used to specify scope in a system.