String and string  are the exact same thing. The .NET Framework
represents strings using the System.String class. In C#, string is an
alias for System.String. That is, the following three lines of C# code
are semantically identical:

string x;
String y;
System.String z;


In a similar vein, int is an alias for System.Int32, so the following
are also equivalent:

int x;
Int32 y;
System.Int32 z;

and so forth.

For more information, check out:
ms-help://MS.NETFrameworkSDK/csref/html/vclrfString.htm
ms-help://MS.NETFrameworkSDK/cpref/html/frlrfSystemStringClassTopic.htm