When one class inherits from another, the child inherits all the behavior and properties defined by the parent. It also has the opportunity either to define its own additional behavior and properties, or override the behavior of the parent.
In the case of Objective-C string classes, the class description for NSMutableString specifies that the class inherits from NSString, All of the functionality provided by NSString is available in NSMutableString, such as querying specific characters or requesting new uppercase strings, but NSMutableString adds methods that allow you to append, insert, replace or delete substrings and individual characters.