Selenium 2 is a combination of Selenium WebDriver and Selenium RC (Selenium 1) which is based on Selenium-Core. Selenium-Core is a JS program encompassing a set of JS actions which interpret and execute Selenium commands. With Selenium 3, the core is replaced with the back-end WebDriver, with Selenium RC indirectly accessible from this WebDriver.
Other differences between the two include:
- Selenium 2's WebDriver provides its own API for interacting with the browser while Selenium 3 requires Browser-specific implementations of the WebDriver, such as GeckoDriver for FireFox. These drivers offer better testing for the platforms they are designed for.
- For Selenium 2, mobile testing is performed with AndroidDriver or iPhone driver, whereas in Selenium 3, tools such as Appium are used for this.
Details on the differences between Selenium 2 and 3 can be found here.