Base class Libraries || Framework class Libraries in .net

Class library logically divided into name space and  will contain types similar to header files in 'c' language..Net provides class libraries but unlike header files a class library in .net will contain types.A type can be a class or structure or enumerator or Interface or a delegate. A class library directly will not contain the types,and it is divided into name space and s will contain that type purpose of the name space is to logically group the related types

When ever you want to use a type available in a class library in your application then you must add the reference of the name space that containing type you want to use to your application and for this purpose vb.net provides "imports" statement and C# provides "using" Statement
And they are similar to # include statement in 'C' language

.Net 4.5 framework provides the assemblies name space and types as follows in different versions please click here