A string is a sequential collection of Unicode characters that is used to represent text. At the other hand the string builder is sealed class(Cannot be inherited)it represents a mutable string of characters.

String is mutable that means that it can't be modified once created, to modify a given string by appending new characters or so the CLR destroy the first string instance and create a new string one and this costs in terms of memory and time process.

 Using the string builder, you create a mutable object that recieves modification without having to be destroyed and reinstanciated for each modifiaction