The right part shows you the file types in table view, again sortable by various criteria (extension, description, size, percentage of usage, number of files).
The bottom half gives you either the default view or the “Sequoia” view. Both show you the usage of your disk in (‘squarified’) rectangles. The biggest rectangles represent the biggest items on your hard disk and are so easily identified. WinDirStat is an almost complete tool only lacking the possibility to save a scan, and maybe a chart type or two.