WebDriver:

WebDriver object represents the browser in Selenium.
Using this object you can control the Web browser.
It is an interface of the org.openqa.selenium.* package.
Upon instantiating the implementations of this class 
the browser will be launched.
FirefoxDrive, ChromeDriver, 
InternetExplorerDriver, SafariDriver, OperaDriver, 
HtmlUnitDriver, RemoteWebDriver are few 
implementations of the WebDriver Interface.