|This article does not cite any references or sources. (February 2013)|
When developing software, testing the software is a mandatory step. One type of testing, unit testing, involves testing the fundamental units of the software. It may be carried out by writing code that tries out the target unit, checking inputs and outputs, one detail at a time. By keeping such automated testing code, programmers can verify that they haven't broken something along the way. Software to manage these tests are often called code-driven testing frameworks.
Various such frameworks have come to be known collectively as xUnit. These allow testing of different elements (units) of software, such as functions and classes. The main advantage of xUnit frameworks is that they provide an automated solution with no need to write the same tests many times, and no need to remember what should be the result of each test. These frameworks are based on a design by Kent Beck, originally implemented for Smalltalk as SUnit. Erich Gamma and Kent Beck ported SUnit to Java, creating JUnit. From there, the framework was also ported to other programming languages, e.g., CppUnit (for C++), NUnit (for .NET). They are all referred to as xUnit and are usually free, open-source software.
xUnit architecture 
All xUnit frameworks share the following basic component architecture, with some varied implementation details.
Test case 
A test case is the most elemental class. All unit tests are inherited from here.
Test fixtures 
A test fixture (also known as a test context) is the set of preconditions or state needed to run a test. The developer should set up a known good state before the tests, and return to the original state after the tests.
Test suites 
A test suite is a set of tests that all share the same fixture. The order of the tests shouldn't matter.
Test execution 
The execution of an individual unit test proceeds as follows:
setup(); /* First, we should prepare our 'world' to make an isolated environment for testing */ ... /* Body of test - Here we make all the tests */ ... teardown(); /* In the end, whether succeed or fail we should clean up our 'world' to not disturb other tests or code */
The setup() and teardown() methods serve to initialize and clean up test fixtures.
An assertion is a function or macro that verifies the behavior (or the state) of the unit under test. Failure of an assertion typically throws an exception, aborting the execution of the current test.
xUnit frameworks 
Many xUnit frameworks exist for various programming languages and development platforms.
See also 
Unit testing in general:
Programming approach to unit testing:
- Kent Beck's original testing framework paper
- Other list of various unit testing frameworks
- OpenSourceTesting.org lists many unit testing frameworks, performance testing tools and other tools programmers/developers may find useful
- Test automation patterns for writing tests/specs in xUnit.
- Martin Fowler on the background of xUnit.