Gems & Bundler

What's a gem?

A gem is Ruby's unit of shared, packaged code — same role as an npm package or a Python wheel. Anyone can publish one; rails itself is a gem, and it depends on dozens more underneath it. Install one manually with gem install some_gem, then load it in code with require "some_gem" — you've already done this with require "json", which happens to ship in Ruby's standard library and needs no separate install.