Answer:   The main differences between system.stringbuilder and system.string are:
o system.stringbuilder is a mutable while system.string is immutable.
o Append keyword is used in system.stringbuilder but not in system.string.

Answer:   StringBuilder is used to represent a mutable string of characters. Mutable means the string which can be changed. So String objects are immutable but StringBuilder is the mutable string type. It will not create a new modified instance of the current string object but do the modifications in the existing string object. The complete functionality of StringBuilder is provided by StringBuilder class which is present in System.Text namespace.