while using only Selenium 1 you instantiate Selenium as -
Selenium selenium = new DefaultSelenium("host IP of Selenium Server", selenium_server_port, "browser", "appURL");
In case of WebDriver you can get WebDriverBackedSelenium as
Selenium selenium = new WebDriverBackedSelenium(driverObject, appURL)
Notice that in case of WebDriverbackedSelenium, you might not be able to use all Selenium 1 API, as it is emulation of Selenium object.
~ seleniumtests.com