Source : Free On-Line Dictionary of Computing

scroll bar
     
         A {widget} found in {graphical user interfaces} and
        used to show and control ("scroll") which portion of a
        document is currently visible in a window.  A window may have
        a horizontal or, most often, vertical scroll bar or both.
     
        A vertical scroll bar is a narrow strip drawn up the side of
        the window containing a "bubble" whose position in the scroll
        bar represents the position of the visible part within the
        whole document.  By dragging the bubble with the mouse the
        user can scroll the view over the entire document.  Arrow
        buttons are usually provided at the end(s) of the scroll bar
        to allow the window to be scrolled by a small amount, e.g. one
        line of text, in either direction by clicking them with the
        mouse.  Some programs provide a second pair of buttons for
        scrolling a page at a time or some other unit.  Clicking on
        the scroll bar outside the bubble will either, depending on
        the particular {WIMP}, move the bubble to that point or move
        it some amount (typically a screenful) in that direction.
     
        Different {WIMP} systems define different standards for
        whether scroll bars appear on the left or right, top or bottom
        of the window, and for their behaviour.
     
        To reduce mouse movement, the up and down scroll buttons
        should either be next to each other at one end of the scroll
        bar (as in {NEXTSTEP}) or should reverse their effect when
        clicked with the right-hand mouse button (as in the {X Window
        System} and {RISC OS}).  The fraction of the scroll bar filled
        by the bubble should indicate the fraction of the document
        visible in the window.
     
        (1998-06-26)