Answer:   BCL is a base class library of classes, interfaces and value types
It is the foundation of .NET framework applications, components, and controls
Encapsulates a huge number of common functions and make them easily available for the developers
It provides functionality like threading, input/output, security, diagnostics, resources, globalization, etc.
Also serves the purpose of interaction between user and runtime
It also provides namespaces that are used very frequently. for eg: system, system.Activities, etc..