I learned about Ruby libraries: writing_hand:
A lot of words came out and it seemed to be confusing, but I summarized it as follows.
: white_check_mark: library </ font>: Program extension. It combines complex programs and versatile programs into one. : white_check_mark: RubyGems </ font>: System that manages Ruby libraries : white_check_mark: Gem </ font>: Ruby library packaged in the RubyGems system format : white_check_mark: RubyGems.org </ font>: A site where many Ruby libraries are collected in Gem format.
I see: point_up:
RubyGems.org is like a treasure trove of Gem with lots of useful Ruby features! Ruby on Rails </ font>, which is also a framework </ font> (a tool for creating a framework and creating an application efficiently and easily), is also available. It turned out to be one of the "Gems" that combine various Gems.
In advancing this learning, I referred to the article by TECH CAMP blog, so I will post a link. When I typed "Ruby" and searched, many articles came out, so I would like to read it again when I have time.
2020-08-16 The page has been updated with the advice of @scivola. Thank you very much.