Ruby Compendium - The Core and the Standard Library

By default, Ruby always loads its core classes and modules when a script is executed. This means that in all Ruby programs you can always instantiate core objects like:

Numbers (integers, floats)
Strings
Arrays
Hashes
Files
Regular expressions
Symbols
Threads
Times and dates
…and many more.

Most likely, this is not going to be enough. That’s when the Standard Library comes into play: it’s a large collection of internal libraries that ships with every Ruby implementations. There are libraries to connect to the Internet, to read/write to various file formats such as CSV or YAML, to work with files and paths, access system features, and so on.

Unlike core classes, standard libraries must be required specifically if needed, before they can be used.