I came across this excellent, brief, and very useful write-up on Code Coffee that explains the DU and DF commands and gives some usage examples.
These commands are both extremely useful for getting a quick handle on your file system or on a particular directory.
I won’t re-invent the wheel, but rather will just give you the link:
http://www.codecoffee.com/tipsforlinux/articles/22.html
Enjoy!