The f-pattern gets mentioned on the web and as you would expect it follows the shape of the letter F. I think Jacob Nielsen first suggested the pattern after eye-tracking studies his company performed. What often gets lost in the f-pattern is that these original studies were done on text heavy designs and search results. As with the other patterns the eye starts in the top/left, moves horizontally to the top/right and then comes back to the left edge before making another horizontal sweep to the right. This second sweep won’t extend as far as the first sweep. Additional sweeps move less and less to the right and for the most part after the second major sweep the eye sticks close to the left edge as it moves downs.
The F-pattern refers to the tendency of a viewer to first scan a horizontal line across the top of the screen (this is why websites often include a long banner or navigation bar across the top of pages). Next, the viewer may look left, vertically, for keywords, graphics, subsections, etc. at which point their eyes naturally tend to move horizontally.