Disk Usage Space - Difference between "du" and "df"

Hello Everyone!

I use a ESX host(3.0.1), and my local disk reached 100% of disk usage space in "/ ".

When i used the "df -h" command, him return for me the status of 100% used. So, I started the search for tempory files that i will can delete for liberate more space.

But, when I use the "du" command, for seek in wich directory of "/ " has more use of disk, him return for me that disk usage just in 1.5Gb, in other words, don´t show me what use my disk space fully.

Someone can help me?

Thks!!!