Debian is a free operating system that uses the Linux kernel and many tools from the GNU Project. It is well known for it's package manager which is refered to as Apt. Debian comes with over 8710 programs all of which are precompiled, and can be installed using Apt.
There are three different distributions of Debian that we will have to worry about for the sake of this article. Stable, Testing, and Unstable. These names do not describe necessarily the quality of the software they use, but how often the packages are updated. Each distribution serves a purpose and is suited for different uses.
Stable
The stable release of Debian is rarely updated. The software available in Woody is generally considered to be quite old, but it is all guaranteed to work well together. It is however updated regularly with security updates, something that the testing and unstable distributions do not receive. Stable is a very good choice if you are running a server.
Unstable
On the other end of the spectrum is the unstable distribution. Unstable is updated regularly with the lastest software releases. Because of the fast moving nature of Unstable, there is the occasional conflict between packages. These problems are usually cleared up very quickly. Unstable is an good choice for a desktop user that must have new software asap.
Testing
In the middle of the road is the Testing distribution. Any packages that have been in Unstable for 10 days are automatically added to Testing. Because of this 10 day wait, Testing is much less likely than Unstable to have any conflicts between packages. Testing would be a good choice if you are running a server, or if you are a desktop user who doesn't need the newest software immediatly.
Installation
Installing Debian is as easy as downloading one CD. Just grab the first ISO file from one of the many mirrors. It is recommended that you use a downloading program that supports resuming to do this. After you have the cd image burn it to CDR using your favorite burning program. After you have your CD, simply boot from the CD and follow the installers instructions.
Post-Installation
After you have Debian installed, you still have to do a few things to get everything up and running. First off you should edit your source.list file to use the Debian distribution (stable, testing, or unstable) most suited to your needs. For example, to switch to the unstable distribution, open /etc/apt/source.list in your favorite editor, and change any instances of "stable" to "unstable".
After you have edited your sources file, you are going to want to updated your system. Run 'apt-get update' to update your list of available packages, adn then run 'apt-get dist-upgrade' to download and install all the new packages.
If you are a desktop user, you are going to want to run a desktop environment or just a window manager. The easiest way to install one of these is using apt. For instance, if you wanted to install the Gnome desktop environment, you would run 'apt-get install gnome-desktop-environment'. That command will download all the necessary packages to get Gnome up and running for you.