Difference between String (Starts with a capital "S") and string (Starts with a simple s)

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

But int automatically references System.Int32 because it is the default, while there are many other integer(int) types. Using int just defaults to the 32 bit integer if you don´t explicitly tell in your code what type of integer it should expect. If you reference Int16 instead of int, you will specify the type from the default Int32.

As far as guidelines, I think it's generally recommended to use string anytime 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);