What is test harness?

test harness: A test environment comprised of stubs and drivers needed to execute a test.

Test harnesses and stubs will be used to replicate the missing items (components not yet included in the tests or external systems). Often, when small-scale Integration Testing of several modules or components is performed, it is necessary to devise or improvise methods and tools to get the test data to the components under test. This is often called a test harness. Because of the need to understand the technicalities required to build a test harness this testing is almost always done by the development team.

A test harness may facilitate the testing of components or part of a system by simulating the environment in which that test object will run. This may be done either because other components of that environment are not yet available and are replaced by stubs and/or drivers, or simply to provide a predictable and controllable environment in which any faults can be localized to the object under test. These are usually bespoke programs generated by developers to help in the testing process. If they are used in a mature organisation it is quite possible that these harnesses will be considered as ‘Test Assets’ and subject to Version Control & Configuration Management.

Test harnesses contains all the information required to compile and run a test. This includes, test cases, source files under test, stubs, and Target Deployment Port (TDP) configuration settings.

Several broad questions there, will try to answer based on my experience.

Think of a Test Harness as an 'enabler' that actually does all the work of (1)executing tests using a (2)test library and (3)generating reports. It would require that your test scripts are designed to handle different (4)test data and (5)test scenarios. Essentially, when the test harness is in place and prerequisite data is prepared (aka data prep) someone should be able to click a button or run one command to execute all your tests and generate reports.

A test harness is most likely a collection of different things that make all of the above happen. If you wrote unit tests while developing your application, that would be part of a test harness. You would also have other tests for the functionality of your app, like: user logs in to site, sees favourites pane, recent messages and notifications. Then you add in a 'runner' of sorts that goes through all of your "test scripts" and runs them (instead of you having to execute tests one at a time). If it feels like a test harness is more of a conceptual collection rather than a single piece of software, then you're understanding this correctly :-)

Now my question is what is the difference between test case and test script?

Simple but not entirely correct answer: A Test Case defines test objectives, description, pre-conditions, steps (descriptive or specific), expected results. A Test Script would then be the actual automated script that you execute to do that test. That's in an Automation context. And it changes. A lot.

What certifications like ISTQB define as test scenarios is usually referred to as test cases in some companies and countries. In others, test cases are flipped with test scripts when referring to manual testing (when the steps are given in detail but not part of an automation harness). Others say that test scripts exclusively mean automated tests. On the other hand, one can also argue that several test cases can be combined in a test script and vice-versa. So that begs the question, how does a test procedure fit in?

A test development stage can have: "Test procedures, test scenarios, test cases, test datasets, test scripts to use in testing software."

If you assume a > (is larger than/collection of) relation, how would you relate those? Rhetorical question - that differs based on where you work, who your client is, etc. Best thing is to define it with your colleagues/clients and agree on the understanding of the terms rather than the definition. I currently go with test script = automated script, based on a pre-existing manual test case or a test scenario.

Also, how do you use the software to test the different functions of the AUT?

You write different tests to test different things. Each test does certain actions and checks if the AUT's output matches what you expected - If displayed_value == expected_value. An input file could be used to provide data for the test- list of test usernames and passwords, for instance. Or run the same test with different data - login as a different user with different messages, etc.

Take a look at RobotFramework and the Selenium. A robot framework test (written in text or html files) combined with the Selenium library would allow you to write an automated test which tests something specific...like a home page validation. You would write a separate test to ensure that a user can see all his/her messages. Another to test clearing notifications. And so on.

A Test Harness is the collection of all the items needed to test software at the unit, module, application or system level and provides the mechanism to execute the test. Every item such as input data, test parameters, test case, test script, expected output data, test tool, and test result report is part of the test harness.