Difference between String & string

"String" is an alias ( the same thing called with different names) of "string". So technically both the below code statements will give the same output.

String s = "C# interview questions";

or

string s = "C# interview questions";

In the same way there are aliases for other c# data type as shown below:-

object: System.Object
string: System.String
bool: System.Boolean
byte: System.Byte
sbyte: System.SByte
short: System.Int16
ushort: System.UInt16
int: System.Int32
uint: System.UInt32
long: System.Int64
ulong: System.UInt64
float: System.Single
double: System.Double
decimal: System.Decimal
char: System.Char

Difference: 
When we talk about .NET there are two different things one there is .NET framework and the other there are languages ( C# , VB.NET etc) which use that framework.

"System.String" => "String" ( capital "S") is a .NET framework data type while "string" is a C# data type.

when we should use "String" & "string":

First thing to avoid confusion use one of them consistently. But from best practices perspective when you do variable declaration it's good to use "string" ( small "s") and when you are using it as a class name then "String" ( capital "S") is preferred.
In the below code the left hand side is a variable declaration and it declared using "string". At the right hand side we are calling a method so "String" is more sensible.

string s = String.ToUpper() ;