Answer:   A string is immutable means you cannot change it after it was created. Any operation that appears to change the string instead returns a new instance whereas when you need a mutable string, such as where you need to change lots of things, you use a StringBuilder which is a buffer of characters that can be changed.

Answer:   String is an immutable object and StringBuilder is a Mutable Object. Performance wise string is slow because its’ create a new instance to override or change the previous value.