Difference between String and string data types

There is no difference between string and String data type. string is C# based keyword and it is alaises for CLR Type(String).

C# defines a number of aliases for CLR types. It have the keywords because they are easier to remember and programmers coming from other languages like c/c++ would also be familiar with these types. They may be used interchangably, and even mixed together,
e.g : string x = new System.String(' ', 5);

These are the aliases defined:

string->System.Strings
byte->System.SByte
byte->System.Byte
short->System.Int16
ushort->System.UInt16
int->System.Int32
uint->System.UInt32
long->System.Int64
ulong->System.UInt64
char->System.Char
float->System.Single
double->System.Double
bool->System.Boolean
decimal->System.Decimal