What are the different Browser specifications in Selenium web-driver?

1. ChromeDriver()
2. InternetExplorerDriver()
3. AndroidDriver()
4. IphoneDriver()
5. FirefoxDriver()
6. HTMlUnitDriver()
7. RemotewebDriver()
8. IphonesimulatorDriver()
9. EventFiringWebDriver()

Out of all the available driver implementations HTMLUnit  Driver is the fastest one.
HTMLUnit Driver does not execute tests on browsers but on plain HTTP request response.
HTMLUnit is also called Headless browser driver.