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. What's the difference between the both classes? this is the question The first one 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