What are the significant changes in upgrades in various Selenium versions?

Selenium 1: Included 3 suite of tools: Selenium IDE, Selenium RC and Selenium Grid. There was no WebDriver in Selenium v1. 

Selenium 2: Selenium WebDriver was introduced in Selenium v2. With the onset of WebDriver, Selenium RC got deprecated and is not in use since. Older versions of RC is available in the market though, but support for RC is not available. 

Selenium 3: Currently, Selenium v3 is in use, and it comprises of IDE, WebDriver and Grid.

IDE is used for recording and playback of tests, WebDriver is used for testing dynamic web applications via a programming interface and Grid is used for deploying tests in remote host machines.