The Pony standard library is a collection of packages that can each be used as needed to provide a variety of functionality. For example, the files package provides file access and the collections package provides generic lists, maps, sets and so on.
There is also a special package in the standard library called builtin. This contains various types that the compiler has to treat specially and are so common that all Pony code needs to know about them. All Pony source files have an implicit use "builtin" command. This means all the types defined in the package builtin are automatically available in the type namespace of all Pony source files.
Documentation for the standard library is available online