BCL (Base Class Library) vs FCL (Framework Class Library)

The Base Class Library (BCL) is literally that, the base. It contains basic, fundamental types like System.String and System.DateTime.

The Framework Class Library (FCL) is the wider library that contains the totality: ASP.NET, WinForms, the XML stack, ADO.NET and more. You could say that the FCL includes the BCL.

BCL:

A .NET Framework library, BCL is the standard for the C# runtime library and one of the Common Language Infrastructure (CLI) standard libraries. BCL provides types representing the built-in CLI data types, basic file access, collections, custom attributes, formatting, security attributes, I/O streams, string manipulation, and more.

FCL:

The .NET Framework class library is exactly what its name suggests: a library of classes and other types that developers can use to make their lives easier. While these classes are themselves written in C#, they can be used from any CLRbased language

You'll be using the BCL with some parts of the FCL with each project type. So System.Windows.Forms (a separate library) or System.Web, with the BCL from mscorlib and System.dll

The BCL is a subset of the FCL. BCL honors the ECMA specification for the common language infrastructure. Then Microsoft added all their goodness like data and xml and called it the Framework Class Library. Basically they took the BCL and made it go to 11!