Source : Free On-Line Dictionary of Computing

bells and whistles
     
        [By analogy with the toyboxes on theatre organs].  Features
        added to a program or system to make it more {flavourful} from
        a hacker's point of view, without necessarily adding to its
        utility for its primary function.  Distinguished from
        {chrome}, which is intended to attract users.  "Now that we've
        got the basic program working, let's go back and add some
        bells and whistles."  No one seems to know what distinguishes
        a bell from a whistle.
     
        [{Jargon File}]