Difference between String and string in c#

string is an alias for System.String. So technically, there is no difference. It's like int vs.System.Int32.

As far as guidelines, I think it's generally recommended to use string any time you're referring to an object. e.g.

string place = "world";

Likewise, I think it's generally recommended to use String if you need to refer specifically to the class. e.g.

string greet = String.Format("Hello {0}!", place);

This is the style that Microsoft tends to use in their examples.