Automatically called by QUnit after each test. Can be called by test code, though usually its better to seperate test code with multiple calls to test().
When QUnit is run in a browser, it looks for #main (deprecreated, but still supported) or #qunit-fixture elements. If found, it'll store the contained markup before running any test, then restoring that markup after each test. If tests manipulate elements only within one of those elements, they won't affect each other.
Currently this depends on jQuery being loaded, as jQuery's .html() method is used for cleanup. Just using .innerHTML isn't enough here.