What is the difference between Use Case, Test Scenario and Test Case?

 Use Case:

        Use Case describes the functional requirements, which help us to understand the client needs.

        It is written by Product Owner / Business Analyst.

Test Scenario:

        Based on the Use Case ,Test Scenario is created and it is Tester responsible.

        Test Scenario --- "What to Test"

        Example : Validate the functionality of Login Button.

Test Case:

        Test Case describes Test steps/ Procedure.

        Test Case --- "How to Test"

        Step by step actions to be performed to verify the functionality.

        It is prepared by Test Engineer.

Example:  

TC1: Click the submit button without entering Username and Password

TC2: Click the submit button with valid credentials.

Simple diagrammatic representation:

          Here Test Document is the Use Case, from Use Case we will derive multiple Test Scenarios, From each Test Scenario we have multiple Test Cases.