Airport Extreme? is it like dropbox? this is the 1st time i heard it. Sorry hehe
With an
Airport Extreme you can share a volume connected to a
Mac over the Internet, like dropbox, yes. The Mac has to have file sharing turned on and you have to map TCP port 548 to the Mac doing the sharing. You must then have account(s) set up for individual people to share the volume, and specifically allow them read, write, or read/write permission on the share.
The Airport Extreme does not have a disk drive, only a Time Capsule does. Time Capsule is an Airport Extreme with a disk, which can also accommodate another disk. Those disks can be shared over the Internet, like drop-box, yes.
I don't know if you can do this with an Airport Express. It too has a USB port, but I don't know if it can accommodate a disk drive. I use my Airport Express to drive a stereo system and have a printer attached to the USB port.
In the pic I uploaded above, see the option to share the disks of a Time Capsule in airport utility. Also be aware if you are going to do this, you will be exposing the disks to the Internet and you will need to password protect the disk(s). There is a good writeup on how to do this here:
How-To: Access Your Time Capsule Over the Internet.