title: Difference between WebDriver and FirefoxDriver
body: <p>I am completely new to Selenium WebDriver. Can anyone please tell me what is the difference 
  between these two lines?</p>
  
  <pre><code>WebDriver driver = new FirefoxDriver();
  </code></pre>
  
  <p>and</p>
  
  <pre><code>FirefoxDriver d = new FirefoxDriver();
  </code></pre>
  
  <p>Both launch the Firefox browser. So Why we always write the first line instead of 2nd line? </p>