A class library is a collection of pre-written code that developers can use to perform common tasks without having to write everything from scratch. These libraries contain classes, which are blueprints for creating objects, and methods, which are functions that define the behavior of those objects. By using class libraries, programmers can save time and reduce errors in their code. Class libraries are often part of larger frameworks, such as Java or .NET, and can be used across different projects. They help standardize coding practices and improve collaboration among developers by providing a shared set of tools and functionalities.