One of the significant features of the .NET Framework is that it comes with an enormous class library called the Framework Class Library (FCL). The types provided by the FCL can be used in any .NET language.

To begin with, you may want to become familiar with the following types:

System.Object
System.ValueType
System.String
System.IComparable
System.IEnumerable
System.Collections.ArrayList
System.Collections.Hashtable
System.Collections.IEnumerator

There are also types for accessing databases, using XML, and many more besides these!