1 "String" with a capital "S" is a keyword that refers to the built-in string data type in the .NET Framework's Base Class Library. It is a reference type that represents a sequence of characters. 2 3 On the other hand, "string" with a lowercase "s" is an alias for the "System.String" type, which means they are essentially the same thing. The use of "string" is just a shorthand way of referring to the "System.String" type, and it is used more commonly in C# code. 4 5 Both "String" and "string" are interchangeable in C#, and you can use either one to declare a variable of type string. 6 7 String myString = "Hello World"; // using the String keyword 8 string myString = "Hello World"; // using the string alias 9 However, it is recommended to use the "string" alias in C# code for consistency with the rest of the language's syntax and convention.