The evolution and architecture of Selenium WebDriver, from its origins as JavaScriptTestRunner in 2004 to the current Selenium 4 WebDriver, are explored in detail, highlighting key historical developments and the intricate hierarchy of its components. Selenium was initially developed to address the need for frequent testing of web applications, leading to the creation of various tools like Selenium Core, Selenium RC, and Selenium Grid. The WebDriver, created in 2006, offers a unified, cross-platform interface for automating web browser interactions. Selenium 4, which is W3C compliant, introduces the Selenium Manager and enhances browser compatibility, streamlining the setup process for automation tests. The RemoteWebDriver class, central to Selenium WebDriver's architecture, is detailed along with its nested classes and interfaces, enabling complex interactions such as handling browser windows, tabs, cookies, and navigation. Understanding these components and their interactions is crucial for leveraging Selenium's full potential in test automation, as emphasized by the author, Faisal Khatri, a seasoned software testing professional.