Difference between concat and append function

The concat function is present in String class but the append functin is present in StringBuffer/StringBuilder class.
The concat function concats the string on which it is invoked with the string passed as parameter to this function. The result returned is a new String with both the strings concatanated.

The append function appends a String to the String represent by the StringBuffer/StringBuilder object on which the append() method is invoked. The result returned is the same StringBuffer/StringBuilder object on which this method was invoked.