A class library is a pre-coded object-oriented programming (OOP) template collection. Both network and desktop applications use class libraries. Class libraries contain code for graphical user interface (GUI) elements such as buttons, icons, scroll bars and windows as well as other non-GUI components.
All OOP languages have class libraries. Languages with .NET framework use Base Class Library (BCL). Java languages use Java Class Library (JCL).
Techopedia explains Class Library
Class libraries enhance code reuse by providing implementations of repetitive jobs. Writing program applications from scratch can be an extremely detailed and expensive process. Class libraries include all essential classes in a previously written coded format, which not only simplifies programming but also increases code quality. Class template customization is implemented according to specific programming requirements.
Class libraries are continuously updated and time tested for reliability to reduce programming language limitations. This is especially true in platform-independent languages like Java.