The word tests can be interpreted in multiple ways. Most often, tests perform the validation and verification of the application requirements. Tests can be performed on multiple levels, covering single methods to whole business features. Tests can also cover nonfunctional aspects such as security or performance.

First, let's introduce categories of tests that validate the functional requirements. Mike Cohn has introduced a concept of the test pyramid, which is shown here:

As you can see, the majority of tests in an application are usually tests that cover units of code. A unit can be a single method, the most basic feature. Because of this scope, these type of tests are called unit tests. They can be defined as tests written by a programmer to verify that a relatively small piece of functionality is doing what it is intended to do. Because the unit is rather small, the number of these tests increases rapidly, so they become the fundament of application testing appearing on the lowest...