First, just a note about re-installations. With Debian, a circumstance that will require a complete re-installation of your system is very rare; perhaps mechanical failure of the hard disk would be the most common case.
Many common operating systems may require a complete installation to be performed when critical failures take place or for upgrades to new OS versions. Even if a completely new installation isn't required, often the programs you use must be re-installed to operate properly in the new OS.
Under Debian GNU/Linux, it is much more likely that your OS can be repaired rather than replaced if things go wrong. Upgrades never require a wholesale installation; you can always upgrade in-place. And the programs are almost always compatible with successive OS releases. If a new program version requires newer supporting software, the Debian packaging system ensures that all the necessary software is automatically identified and installed. The point is, much effort has been put into avoiding the need for re-installation, so think of it as your very last option. The installer is not designed to re-install over an existing system.
Here's a road map for the steps you will take during the installation process.
Back up any existing data or documents on the hard disk where you plan to install.
Gather information about your computer and any needed documentation, before starting the installation.
Create partitionable space for Debian on your hard disk.
Locate and/or download the installer software and any specialized driver or firmware files your machine requires.
Set up boot media such as CDs/DVDs/USB sticks or provide a network boot infrastructure from which the installer can be booted.
Boot the installation system.
Configure one network interface.
Open an ssh connection to the new system.
Attach one or more DASDs (Direct Access Storage Device).
Create and mount the partitions on which Debian will be installed.
Watch the automatic download/install/setup of the base system.
Install a boot loader which can start up Debian GNU/Linux and/or your existing system.
Load the newly installed system for the first time.
If you have problems during the installation, it helps to know which packages are involved in which steps. Introducing the leading software actors in this installation drama:
The installer software,
the primary concern of this manual. It detects hardware and loads
appropriate drivers, uses
dhcp-client to set up the
network connection, runs
debootstrap to install
the base system packages, and runs
tasksel to allow
you to install certain additional software.
Many more actors play smaller parts in this process,
debian-installer has completed its task when
you load the new system for the first time.
To tune the system to your needs,
tasksel allows you
to choose to install various predefined bundles of software like a Web
server or a Desktop environment.
One important option during the installation is whether or not to install a graphical desktop environment, consisting of the X Window System and one of the available graphical desktop environments. If you choose not to select the “Desktop environment” task, you will only have a relatively basic, command line driven system. Installing the Desktop environment task is optional because in relation to a text-mode-only system it requires a comparatively large amount of disk space and because many Debian GNU/Linux systems are servers which don't really have any need for a graphical user interface to do their job.