Webdriver is basically an Interface which is always implemented by any of browser class like ChromeDrive() , FirefoxDriver(), OperaDriver() etc.

WebDriver is an interface which can be implemented by the class which implements it. ChromeDriver,geckoDriver,IEDriver these are the class which implements the webdriver interface . Since webdriver is an interface it contians all the abstract methods .These methods are implemented by the class implementing the interface . Hence in our code we write WebDriver driver = new ChromeDriver(); we can also write ChromeDriver driver = new ChromeDriver(); But here we can only invoke methods that are of chrome driver. Whereas in WebDriver driver = new ChromeDriver(); we can invoke the methods of both chrome driver and webdriver