The NSMutableString class declares the programmatic interface to an object that manages a mutable string — that is, a string whose contents can be edited — that conceptually represents an array of Unicode characters.
To construct and manage an immutable string — or a string that cannot be changed after it has been created — use an object of the NSString class.
The NSMutableString (nsmutablestring) class adds one primitive method
-replaceCharactersInRange:withString:
to the basic string-handling behavior inherited from NSString nsstring. All other methods that modify a string work through this method.
More information : NSMutableString Class reference
