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.
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.