The original version of Selenium
Selenium 2 was a rewrite of Selenium 1 that was implemented with WebDriver code.
Selenium 3 was the implementation of WebDriver without the Selenium RC Code. It has since been replaced with Selenium 4, which implements the W3C WebDriver specification.
Selenium IDE was the original Firefox extension for Record and Playback.
The endpoints and payloads for the now-obsolete open source protocol that was the precursor to the W3C specification.
These capabilities worked with the legacy JSON Wire Protocol
Information of interest to developers of Selenium