The string type represents a sequence of zero or more Unicode characters.
string is an alias for String in the .NET Framework.

http://msdn2.microsoft.com/en-us/library/362314fe(VS.80).aspx

C# has defined its own keywords for the basic types. They all compile to
the type in the BCL that corresponds.

Other such keywords are:

int -> Int32
short -> Int16
ushort -> UInt16
bool -> Boolean
float -> Single
etc.

Note that string and String is 100% identical as string is compiled as
String.

The only place that I know of that one of them can be used and not the
other is when specifying the size of an enum like inheritance:

public enum SomeEnum : int

in this case it is apparently not legal to use Int32 instead of int, for
some odd reason.

Actually C# doesn't understand a String type.  It does understand a
System.String type, and you can refer to that as just String, if you
happen to have a
 using System;
statement at the top of your file (and it is some time since I saw a
C# file that /didn't/ have such a statement).

C# does understand the 'string' type, which it internally converts to
System.String32 (much as it converts 'short' to System.Int16).

My personal preference is to always use the C# names, unless I want to
emphasize the exact length of an integer (in which case, I'll use
System.Int64 or whatever).