The biggest change in Selenium recently has been the inclusion of the WebDriver API. Driving a browser natively as a user would either locally or on a remote machine using the Selenium Server it marks a leap forward in terms of browser automation.
WebDriver is a tool for automating testing web applications and in particular to verify that they work as expected. It aims to provide a friendly API that’s easy to explore and understand easier to use than the Selenium-RC (1.0) API, which will help make tests easier to read and maintain. It’s not tied to any particular test framework, so it can be used equally well in a unit testing or from a plain old “main” method.
Selenium WebDriver is the successor to Selenium RC. Selenium WebDriver accepts commands (sent in Selenese, or via a Client API) and sends them to a browser. This is implemented through a browser-specific browser driver, which sends commands to a browser, and retrieves results. Most browser drivers actually launch and access a browser application (such as Firefox or Internet Explorer); there is also a HtmlUnit browser driver, which simulates a browser using HtmlUnit.
Unlike in Selenium 1, where the Selenium RC server was necessary to run tests, Selenium WebDriver does not need a special server to execute tests. Instead, the WebDriver directly starts a browser instance and controls it. However, Selenium Grid can be used with WebDriver to execute tests on remote systems