1.A group of generic collections—in a logical hierarchy by function—that enable you to access the core functionality you need in your applications
2.A group of methods—in a logical hierarchy by class—that enable you to access the core functionality you need in .NET
3.A group of assemblies—in a logical hierarchy by function—that enable you to access the core functionality you need in your applications
4.A group of classes, structures, interfaces, enumerations, and delegates—organized in a logical hierarchy by function that enable you to access the core functionality you need in your applications