The major difference between Selenium 2 and Selenium 3 is, Selenium 3 has bug fixes from selenium 2. Also, Selenium 3 is more mobile automation focused.
Here is the summary of the change from Official Selenium Blog.
- For WebDriver users, it's more of bug fixes and drop-in replacement for 2.x
- Selenium Grid bug fixes are done as well.
- Selenium project will not actively support only the WebDriver API.
- The Selenium RC APIs have been moved to a “legacy” package.
- The original code powering Selenium RC has been replaced with something backed by WebDriver, which is also contained in the "legacy" package.
- Mozilla has made internals of Firefox browser more stable. So community provided Firefox Driver will no longer work. That means that from Firefox 48 you must use their geckodriver to use that browser, regardless of whether you're using Selenium 2 or 3.