The du command provides a summary of disk usage for files and directories. The default behavior is to show the number of blocks used by the contents of a directory or directories the command is run on. Usage is calculated recursively for directories. When du encounters a directory it will recurse into subdirectories and show the disk utilization of the files and directories under that directory and then present a total for the topmost directory. This cascades down through each subdirectory where the subdirectory becomes the parent and each child directory is summarized and the parent then totalled.
For complete show notes see http://www.linuxintheshell.com/2012/07/17/episode-011-du-disk-usage/