Network connection. I want to thank you guys for helping me on this. I have twin girls and I haven't been able to do too much online or anything.
From memory you can format any drive as an OSX extended disk and plug it into the USB port on the airport to use it as a time machine drive. If you want to connect straight to a network drive look at this page - it may help.
I did something similar a few years ago but ended up changing to a program to copy areas of my mac to a folder on my home server which is then set to back itself up anyway.
Hope it helps