The difference between String and StringBuilder in .NET

Reference MSDN
A String object is called immutable (read-only) because its value cannot be modified once it has been created. Methods that appear to modify a String object actually return a new String object that contains the modification. If it is necessary to modify the actual contents of a string-like object, use the System.Text.StringBuilder class.
StringBuilder class represents a mutable string of characters. This class cannot be inherited. This class represents a string-like object whose value is a mutable sequence of characters. The value is said to be mutable because it can be modified once it has been created by appending, removing, replacing, or inserting characters. Most of the methods that modify an instance of this class return a reference to that same instance. Since a reference to the instance is returned, you can call a method or property on the reference. This can be convenient if you want to write a single statement that chains successive operations one after another.