This class library is a huge library of reusable classes, and value types that are available to all the .NET programming languages.
This class library consists of a hierarchy of namespaces, and each of these namespaces is a logical grouping of classes, structures and enumerations.
We can also divide this namespace into System. Collections. Array List which defines the Array List Type.
So, the root namespace is System under which all the classes resides.
Some classes under System are:-Math, Object, String, Array etc.
For Example:- System. Collection namespace contains classes that defines various collections of objects like:- lists, queues, hash tables, and many data structures.