A plug-in is custom code that you write and attach to a Web test or an HTTP request within a Web test. When you create a plug-in class, you inherit from the corresponding base class in the Web test framework. These classes are WebTestPlugin and WebTestRequestPlugin.
You can use Web tests plug-ins to isolate and reuse code outside the main declarative statements in your Web test. This lets you call custom code as the Web test is run. The Web test plug-in is run one time for every test iteration.