WinDirStat reads the whole directory tree once and then presents it in three useful views:
The directory list, which resembles the tree view of the Windows Explorer but is sorted by file/subtree size.
The treemap, the treemap represents each file as a colored rectangle, the area of which is proportional to the file's size. The rectangles are arranged in such a way, that directories again make up rectangles, which contain all their files and subdirectories. So their area is proportional to the size of the subtrees.
The extension list, which serves as a legend and shows statistics about the file types.
