Debian GNU/Linux is distributed freely over the Internet. It is possible to install it in several ways: from a CD, DVD, USB drive or Blue-ray disk, over a network, bootstrapped from within another Linux distribution, or from a MS Windows system. After the initial installation has been completed, further upgrades and maintenance are performed using built-in package management tools. It's possible to upgrade major software components, or even transition between releases of Debian without reinstalling the system.
How do I get Debian installation media?
Generally one obtains the Debian Installer on a release medium (such as a CD) and uses it to install the desired set of packages, either from the same medium or via a network. There are alternatives such as booting from a network or installing from within an existing system using Debootstrap.
Download and burn a network install CD
For those with broadband Internet access it is often preferable to use a minimal network install CD, rather than download a full set of 650 MB ISO images. This network install ISO (called "netinst") contains just enough packages to install a very basic Debian system. During the install procedure only the required packages are downloaded from the Internet, making it more efficient overall. A netinst iso image is around 180 MB. Also available is the smaller "Business card" size CD image that is around 40 MB.
If your wireless network card is not recognized by the installer you might consider doing the installation using a wired (ethernet cable) connection or installing using a different network card. In any case it may be preferable to install with a wired connection for bandwidth reasons or simply to defer wireless setup until after the initial installation.
If you have no way of establishing a network connection during the installation, then you will need to install using full installation CDs or DVDs.
Download and burn full a CD/DVD set
The full CD set can be downloaded in .iso format from:
http://www.debian.org/distrib - Official download page
http://www.debian.org/CD/http-ftp - Debian mirrors
If you want to download one or more of the full 650 MB iso images, please use the jigdo tool which spreads out the load among various Debian servers, lightening their load as well as giving you a faster download.
Although there are over 30 CDs (or 5 DVDs) in a full set, only the first CD is required to install Debian. The additional CDs are optional and include extra packages, that can be downloaded individually during the installation, or later.
DebianCDContents to find out what is on each CD.
Buy a set of CDs
The easiest way to install Debian is with a set of CDs bought from a vendor. There is a list of CD vendors:
Netboot via another Debian system
There is also the possibility to boot from another computer without needing to use CD/floppy media at all. All you need is a TFTPD and a DHCPD running on the install-from computer. This is called installing via "netboot".
PXEBootInstall - the manual for setting up a netboot server.
An updated installer that supports the newest hardware
DebianLive - Debian Live CD / DVD / USB
InstallFAQ - Including how to install unstable (sid).
FAI, Fully Automatic Installation
PackageManagement - Managing your system
Debian GNU/Linux Installation Guide (all architectures, stable branch)