I don't. Enlighten me (please!)
dd is a built-in Unix command line tool that copies data from an input file/device to an output file/device. Most Unix machines have some predefined "devices" with names like /dev/null (is always null), /dev/zero (is always zero), and /dev/random (essentially a pseudo-random value). They're useful for various things; for example a lot of times people will dump script output they're not interested in to the device /dev/null because it just discards it without displaying it. But you can also use these devices as input (rather than output), such as using dd to write all zeroes to a disk by copying /dev/zero to the disk; or you can write (more or less) random data to it by copying /dev/random to it. /dev/random is quite slow though since it has to be called once for each byte to be generated (fyi there's another device /dev/urandom that doesn't have this liability). For speed's sake, a lot of these "disk nuke" tools will just write all zeros, followed by all ones, etc. - much faster than a random number call.
The main problem with tools like dd is they are very powerful; and if you're not careful you can overwrite all the data on your disk. That's not a problem if you're on the computer you want to nuke, and it's only got one disk in it; but let's say you had TWO disks and only wanted to nuke one of them.
(I've never done that, but I have done "rm -Rf" when I thought I was in one directory but was actually in another...). I personally think it's better to just download a free tool like Boot and Nuke onto a floppy or CD, and use that - the programmer takes care of the bookkeeping.
Oh - ALWAYS BACKUP FIRST!!
A good way to learn about things like dd is to use the "man" command from the command line in Terminal (or iTerm or whatever) - typing "man dd" will show the commands manual page, which tells you how it works, tells you what arguments it accepts, etc.
