Tests are functions which make logical assertions. If all assertions are correct then the test passes, if at least one assertion is incorrect then the test fails. Tests are a useful metric for deciding if an application has met its requirements.

Unit tests test a single piece of functionality, this functionality is delivered by a single unit of code such as a method. The philosophy behind unit tests is that if the functionality of the smallest units of the program can be guaranteed, then it is significantly more likely that the project as a whole is succeeding in delivering its functionality.

For more information on Unit testing see our guide.