If you're like most Android users, you may run into slight space issues when it comes to available internal storage for apps. The daunting task you face, then, is to go through each of your apps and locate the major megabyte offenders. What a pain.
Alternatively, you may discover that your precious SD card is suddenly refusing to write a new episode of the Stuff You Should Know podcast (which is awesome), so now you have to drop everything to figure out where the fat is.
DiskUsage is really quite a simple application: upon being opened, you are prompted to select a storage location to view (internal storage or SD card).