四个部分：

test fixture

A test fixture represents(代表) the preparation needed to perform one or more tests, and any associate cleanup actions. This may involve, for example, creating temporary or proxy databases, directories, or starting a server process.

test case

A test case is the individual(独立的) unit of testing. It checks for a specific response to a particular set of inputs. unittest provides a base class, TestCase, which may be used to create new test cases.

test suite

A test suite is a collection of test cases, test suites, or both. It is used to aggregate(集合)  tests that should be executed together.

test runner

A test runner is a component which orchestrates(安排) the execution of tests and provides the outcome to the user. The runner may use a graphical interface, a textual interface, or return a special value to indicate the results of executing the tests.