The document discusses unit testing and provides definitions, structures, and examples. It defines a unit test as a piece of code that invokes another piece of code and checks assumptions afterward. If the assumptions are wrong, the unit test fails. It discusses the characteristics of good unit tests like being fast, isolated, repeatable, and self-validating. The document also covers unit testing structures like arrange, act, assert and triple A testing as well as tools, techniques, and limitations of unit testing.