About the Gemified Standard Library
Ruby's standard library is in the process of being gemified. More and more libraries will be turned into RubyGems, which can be updated independently from Ruby.
There are two different kinds of standard gems:
Default gems: These gems are part of Ruby and you can always require them directly. You cannot remove them. They are maintained by Ruby core.
Bundled gems: The behavior of bundled gems is similar to normal gems, but they get automatically installed when you install Ruby. They can be uninstalled and they are maintained outside of Ruby core.
About the stdgems.org project
This repository contains three JSON files:
The first two files contain data about all default and bundled gems and which Ruby version comes with which version of the gem. The third file contains data about all the non-gemified standard libraries.
Go to stdgems.org to easily browse the standard gem data and more info.