If you want your own set of Debian ["CDs"] you can burn your own set. This first entails obtaining an ISO image and then burning that ISO image to a blank CD. Before jigdo, there was a worse way : downloading the entire ISO. Debian get updated often. Your ["ISOs"] may become outdated the same day you download. So if you want up-to-date ISO images, you must download a new set of ISO images every day. Clearly, this is not the way you want to obtain Debian ["ISOs"]!
The canonical method of getting Debian ISO images is with jigdo.
Jigdo (which stands for "Jigsaw Download") was written by Richard Atterer and is released under the GNU GPL. It's a tool that allows efficient downloading and updating of an ISO image. Any ISO image. Jigdo is not Debian specific, however Debian has chosen it to be the official method of downloading ISO images.
A common misconception is that jigdo creates ISO images; it doesn't. Let's discuss the overall process of how jigdo allows you to obtain an ISO image. Let Adam be the person offering the ISO image (perhaps he's the Debian release manager). Let Betty be the person who wants to download the ISO image (perhaps she's a Debian user).
1.The first step is that Adam creates an ISO image suitable for burning a CD. He might use a utility like mkisofs or debian-cd to create the ISO image. He also creates two files associated with his newly created ISO image: a .jigdo file and a .template file. He makes these two files available for download to anyone who wants to obtain his ISO image.
2. The second step is that Betty downloads the .jigdo and .template files. She then uses jigdo-lite along with these two files to download Adam's ISO image.
The jigdo tool comes with two utilities: jigdo-file and jigdo-lite. Jigdo-file is used by Adam to create the .template and .jigdo files from the ISO image he wants to offer. Jigdo-lite is used by Betty to download the image using the .jigdo and .template files. If all you want to do is download Debian ["ISOs"], you'll only be using jigdo-lite. You can forget that jigdo-file even exists.
Jigdo addresses all the problems with the other methods of obtaining Debian ISO images:
- It's much faster than downloading the entire ISO image.
- Unlike downloading the entire ISO image, it can take an outdated CD (or a loop mounted outdated ISO image), download only the files that have changed since the CD (or ISO image) was created and create a new updated ISO. Very similar to how you use cvs to update source code.
- jigdo-lite uses wget which, by default, uses http to transfer files. Unlike rsync, http is never blocked by firewalls (except the ones behind which you shouldn't be using jigdo to begin with).
- Jigdo is very kind to the bandwidth of the servers offering the Debian images. The Debian mirrors can handle a bigger load of people using jigdo to download Debian images than with other methods of getting them.
Clearly, jigdo is the best method of obtaining Debian ISO images.