The simplest way to get Debian is to download an ISO file, and burn it onto a CD or USB. Depending on your needs and processor architecture, however, you might need a different ISO.
The standard way to go about installation would be to download the main DVD iso, which contains everything you need, and a lot of what you don't (but others might). There are CD and DVD sized images available, for various processor architectures. Besides the 32 bit, and 64 bit Intel/AMD processors (dubbed i386 and amd64, respectively), Debian is available for arm (-64, -el, -hf), mips (and -64el), ppc64ei s390x, and source architectures as well. If you don't know what these are, you most likely don't need those images.
This guide will focus on the common 32 and 64 bit PC architectures, for which CD and DVD images are available, that can also be directly written to USB sticks.
The official Debian website's download CD/DVD images page, has many options, and information written in plain text, with no pretty images to illustrate, it can be intimidating at first, but even when you get used to it, it might feel overwhelming (it does to me, after many years still).
Still, it's probably best to read through it all, to have a better idea about your options. the website offers ways to download images using HTTP/FTP, jigdo and BitTorrent, and you can even buy finished CD-ROMs (which is a great way to support an otherwise free (as in gratis) project).
Debian encourages using download manager tools. It makes sense. the files are large, and downloads might fail for any number of reasons. Debian's resources are limited, although it might seem to an have enormous infrastructure with some 300 mirrors worldwide, the fact is Debian does not own these servers. Storage space and bandwidth is mostly donated (yes, one can donate stuff too, not only money), but owned by others.
This means that Debian devs and maintainers are quite unwilling to let people waste resources, and rightfully so. Each mirror stores about 100GB(!) of archives, and each failed/paused and restarted download will have wasted many GB of valuable bandwidth.
Download tools have been around since as long as we have stuff to download, or networks to download them from. these tools can manage downloads better than browsers, theí can stop/pause and restart/resume a download. Yet people rarely use them, even for downloading DVD sited images. Go figure. Anyway, if you insist on downloading via HTTP/FTP, you should consider using one of the recommended tools, (or any other tool of your choice). better for you, and better for whoever is paying to maintain the download mirror you are using.
For Unix/Linux, Debian maintainers recommend aria2, or wxDownload Fast or (on the command line) wget -c <URL> or curl -C - -L -O <URL>.
Another option is to download live images via HTTP, FTP or BitTorrent, allowing you to test a system without installing. The idea of a live environment is that you burn it onto a DVD or copy it to a USB drive, and boot from the removable media instead of the hard disk. Debian will run from your media without needing installation. This way you can see if you like it, if your hardware is supported and if it's really the "flavour" you were looking for, as live images offer a glimpse at different desktop environments (currently Gnome, KDE, LXDE, Xfce, Cinnamon and MATE), with a set of conveniently pre-installed packages. You can also install a live environment to disk with a few clicks.
You will find more detailed instructions regarding live images under "Try a Live CD"
CD images are smaller ISO files that fit onto a 650MB CD-R(W) or pendrive. They offer a minimal amount of packages and maximum one desktop environment (Xfce), everything else needs to be pulled in from the internet during installation.
You will be able to choose a different Desktop Environment during installation if you enable using a network mirror. If you do an offline install, you will be limited to he single DE (Xfce if you download from HTTP/FTP/BitTorrent), but will still be able to install a Desktop Environment from the Debian repositories later.
The "netinst" installers under CD images are even smaller, offering even less, and relying more on your internet bandwidth. If you have a fast internet connection, these are the least wasteful, as you'll only download what you really need. If you have slower internet, however, they can take the installation process painfully slow.
Debian DVD images offer a more complete collection of packages. There are quite a few images available (a few on the download sites, and more via jigdo (see above)), which together host every package for Debian. Like all of them. Typically you would only need the first DVD to install a complete system, unless you are creating a monster, or needing some really specific stuff.
DVD images are 4.4GB and take a lot longer to download (and waste a lot more of Debian's server bandwidth if you download via HTTP/FTP), but in certain cases, they make more sense. If you want to install Debian on a computer that is not connected to the internet (do those still exist outside of Nuclear power plants?), or are at a remote location, where internet is slow, unreliable, or unavailable (think rural Africa, or the top of the Himalaya, etc.), it makes sense to prepare a DVD image, and have it handy for easy installations.
If you choose a reliable download method that can resume intermittent downloads (i.e a download manager, jigdo, or BitTorrent), and have enough patience, you can obtain the image once, and re-use it as many times as you need. If you have all the DVD images you need, you will be able to install Debian without having an internet connection.
Alongside the official Debian images, there signed checksum files to verify authenticity and integrity. These can be used to make sure the downloaded images are not broken, and that nobody had tampered with them (e.g. US intelligence agencies are notorious of spying on their own citizens, building back-doors into software, etc. You know the land of the free seems to mean they are also free to know what you do with your freedom. Never mind.)
Verifying a checksum is quite simple and straightforward, but validating GPG signatures can seem quite difficult and intimidating. The good news is, if the signatures on the checksums were off, someone (who knows what they are doing) will most possibly notice it and take appropriate steps. You are mostly safe by verifying checksums only, but make sure you do not skip that simple, yet important step.
Other ways to get Debian
Yes, well, it would be too easy to just download stuff, wouldn't it? For those of you with masochistic tendencies, or if you feel the need to prove to yourself, or anyone around you how very smart and tech-savvy you are, there are some screwed-up, albeit sophisticated(ly overcomplicated) ways to do a simple task normal people do with a few clicks.
Debian is huge. The current release "Stretch" fits onto some 14 DVDs, and with so many supported processor architectures, it would take a lot of storage space to host all pre-built images. For this reason, and because people seem to be unable o grasp the concept of download managers, wasting a lot of bandwidth with restarting failed/interrupted downloads, only the first few DVDs are available on the HTTP/FTP and BitTorrent mirrors. If, for any reason, you'd need the full DVD set downloaded, your only real choice is using a tool called "jigdo" (short for jigsaw download).
Jigdo is a great idea... on paper (or screen). It would take a .jigdo file, that contains information about the sort of image it needs to create, then ask for a Debian mirror to download from, and download the individual files, which would make up an image, finally assemble the image from the downloaded files. So convenient. When it works. and if you have bucket loads of patience. and considerable command-line experience. And understand how it works. And read all its docs. And so on.
The truth is, in an attempt to save themselves hard work, Debian maintainers and developers basically just threw it all onto the user. Now it's up to you to put together the pieces of the puzzle and make jigdo work. To make things worse, jigdo is now in "maintenance mode", meaning it's GUI front-end was never finished, and it's not actively developed anymore. (Windows 10 users will find it particularly difficult to use, and might need to check forums, etc.) That, and also the fact that it was never exactly a painless experience to use jigdo. Just saying'.
Wait you thought it's called "jigsaw download", because of the clever way it assembles downloaded files? That might seem to be the case at first, but the reality is, its name reflects the fact that jigdo can be as puzzling, as a 10k piece jigsaw, especially for someone new to Linux (yeah, bad pun, I know).
The good news is, if you have a decent enough, and reliable internet connection, you don't even need this stuff. You can just download the smallest installation image (around ~250MB usually), and let the installer download packages at installation time (see above). If you do however need to download more than 3 DVDs for some reason, you might be up for a bumpy ride.
The following hidden message may contain images of small and squishy apertures (Microsoft Windows (tm) (r) (NSA) (etc.)). Attempting to follow the instructions can lead to instant and painful hair-loss (tearing your hair out in frustration). You have been warned.
There is Jigdo "mini" how-to accessible on Debian website (although whoever first labelled it "mini" clearly has issues with identifying sizes). The document is a perfect primer for using jigdo. It's long, slightly confusing at first, but might be quite useful, when you get through the initial shock and horror.
If you are on Windows, and feeling adventurous, but cannot bother to read through all of that, here's a short introduction to how jigdo works (if you are on Linux, you are on your own. Sorry, but jigdo is not for the clinically sane to use, and your author could not be bothered to do the screenshots twice. besides if you are on Linux, you might be a little more tech savvy, etc.)
I have warned you before, but I'll do it again. Jigdo is not for the faint-hearted. You will use obscure command line tools, and encounter errors. You will suffer mightily.
Download the Jigdo installer from the official website, and extract the archive on your PC.
Start the batch file called jigdo-lite.bat (just double-click it. Or, alternatively, open a command prompt, navigate to the file and run it like in the good old DOS days). You will be greeted with a similar message:
Now you need a URL. There is a list of URLs for jigdo files on the Debian website. trying any of those will most likely fail. Why? Because those are the official files, listed on the official websites, but jigdo is not supposed to be easy. That is why. So find an official mirror closest to you, navigate to current/, choose your CPU architecture, i.e. amd64/, or i386/ and select either jigdo-cd/ or jigdo-dvd. So e.g. on a 64 bit machine, downloading a CD image, you would be in ftp.your.mirror.domain/current/amd64/iso-cd/. There you can find the jigdo file of your fancy.
Right click on your preferred jigdo file, and select "Copy Link Location" (or something to that effect).
Back to command-line, where right click might or might not work. If it does not, right click on the window border and select Edit > Paste. Otherwise jut paste normally. Only do this if you do not want to manually type the URL. (Why would you?)
Next, you will be asked if you have a previous version of the CD or DVD, because jigdo can, in fact, update images (told ya, it is useful, once you learn how to use it). As you are reading a tutorial on how to use jigdo for the first time, let us assume you do not have one, so just press Enter.
You'll be asked another silly question, namely to specify a Debian mirror. You know, it's not enough that you've already used a mirror to download the jigdo file, you'll have to specify it again, but this time it would be a slightly different URL. Remember, it's not supposed to be easy. It's supposed to make you feel like a sys-admin or something (although I suspect seasoned sysadmins are smarter that, and use far simpler methods to obtain an image).
To find your favourite mirror, just refer to ftp://ftp.debian.org/debian/README.mirrors.txt, or simply press enter, in which case your command line will be flooded with so many mirror addresses, you cannot even scroll up far enough to see all of them. How ergonomic!
Now specify your mirror, press enter, and watch the miracle happen. or possibly the new, more sophisticated,m and even more frustrating error message, such as "Jigdo failed with code xxx - aborting". But that is another story already. Happy debugging.
And that is it. You should soon have an image downloaded, and assembled, which you can update anytime. Or not. You can tell jigdo is my least favourite part of Debian, clearly. I personally avoid it like plague.
Installing from the Internet
You've got to be kidding me. Yes, I know, this is also an official way to get Debian, but
It's not strictly a way to obtain an installation image, as some of the installation steps will also differ,
This document is well over 2000 words long already, and my fingers hurt, and
If you wanna do fancy stuff like that, you can do your homework, and read the official documentation.
This site is for beginners, and people with less technical knowledge. Now get over to the real website, and read the proper docs
Now that you have a Debian image, it's time to make it bootable. Excited yet? Keep reading, you'll see...