Selenium-WebDriver & HTMLUnit

       In my paper，I need  to distinguish the difference between Selenium-WebDriver and HtmlUnit.So google that! The answer comes from stackoverflow:
      "HtmlUnit is a java based implementation of a WebBrowser without a GUI and a way to simulate a browser for testing purposes and Selenium-WebDriver makes direct calls to the browser using each browser’s native support for automation. we can see that HtmlUnit provides API without GUI possibility for automation whereas WebDriver provides internal browsers' possibilities for automation."
       Again,I refer to the official documents of Selenium-WebDriver.
https://code.google.com/p/selenium/wiki/GettingStarted.The page illustrate a test by HtmlUnitDriver .so ,I think Selenium-WebDriver is relatively independent tool.It can test web by invoke many different local browsers,which include FF,Chrome,IE and even the HtmlUnit(Although it doesn`t have a GUI).

    A other reference comper the performance of the browers by grouping the browers into GUI browsers consisting of IE\FF\Chrome and HtmlUnit Driver.the start-up process of the former is very slowly but  the ability of supporting  JS is considerably powerful.the latter is conversely and if the  JS has been more complicated ,the driver can`t capture the element.