Ruby has two major bits of code that it provides for your use:
1. the core library
2. the standard library
The core library can be found here, and contains classes that you know and love, like String, Hash, and Array.
The standard library can be found here, and contains your favorite hits, like CSV, JSON, and Logger.
One way to think about the difference between core and the standard library is that core is written in C, while the standard library is written in Ruby.
Core are the classes that are used the most, so they’re implemented in as low-level a fashion as possible. They’ll be in every single Ruby program, so might as well make them fast!
The standard library only gets pulled in by bits and pieces; another way of thinking about the difference is that you need to require everything in the standard library, but nothing in core.