The BCL is a subset of Framework class library (FCL). The BCL define types that can be used to build any type of software application.  

If we can build a web site or any WebApplication or any REST etc. services using ASP.NET, we can distribute the GUI Distributed systems for building applications use WCF to build WPF, and so on.

This is stands for the Base class library also known as 'Class library' (CL). The BCL is a subset of Framework class library (FCL). The CL(Class_Library) is the collection of reusable types that are closely integrated with CLR. The BCL(Base class library) provides classes and types that are helpful in performing day to day operation e.g. the dealing with string and primitive types, database connection, IO operations.