Debian is composed of free software mostly carrying the GNU General Public License. The operating system is developed by an internet collaboration of volunteers aligned with The Debian Project.
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Pro Extremely easy to find help with any problem
Debian is one of the oldest and most popular distros out there. Debian's popularity means that you will always be able to find a solution for your problem just by searching on Google, or if by chance nobody has had the problem you are having it's very easy to ask the community and quickly get a solution.
Pro Wide choice of hardware platforms
Debian has the widest choice of hardware platforms, including:
amd64, armel, armhf, i386, ia64, kfreebsd-i386, kfreebsd-amd64, mips, mipsel, powerpc, sparc, s390, s390x, source, multi-arch.
Pro Wide choice in desktop environments
Debian offers stable and testing CD images specifically built for GNOME (the default), KDE Plasma Workspaces, Xfce and LXDE. Less common window managers such as Enlightenment, Openbox, Fluxbox, GNUstep, IceWM, Window Maker and others can also be installed.
Pro Standard vanilla Linux desktop
Debian runs standard Gnome, XFCE, KDE - it doesn't use its own special desktop environment, which means that users benefit from the work of the whole Linux community, Debian developers can focus on the distribution itself, and any support for your desktop environment on other distributions should work on Debian as well.
Pro Comes with over 37,500 packages
Because of its popularity, Debian has a lot of applications available which range from productivity programs to business software, games and development tools. It comes with over 37,500 packages (software that is precompiled and ready to be installed on a local machine) -- all of them for free.
Pro Highly secure system
Known for being consistent in maintaining a highly secure system. Several other popular distros use Debian as a base or core for their own Linux OS, the security being one of the main factors why it's so commonly used.
Pro Live install images for CDs, DVDs and USB thumb drives
Debian releases live install images for CDs, DVDs and USB thumb drives, for the i386 and amd64 architectures, and with a choice of desktop environments. These Debian Live images allow the user to boot from a removable media and run Debian without affecting the contents of their computer.
Pro Well-working team
The developer works very well so the the code is one of the best written out there.
Pro Very stable
Pro Rolling or Release based
Every Debian Release can either use a Rolling or Release based model.
Pro A lot of ways to install software
You can either use aptitude, apt or apt-build to install software from the Debian package archives.
Pro Great for gaming
As Debian is the base for SteamOS and because Ubuntu is based on it, it's almost certain that all Linux games will run properly and require no hacks like creating symbolic links because of some hardcoded paths.
Con No default support for non-free hardware
Non-free hardware will not work upon install of Debian and the user will need to search out the correct non-free software to install in order to make them work. This can be difficult if one of the things not working is the WiFi connection.
Con No choices
Debian basically doesn't give you any choices: for example it is very hard to use a different Sound system than ALSA like OSS4, and the same goes for different init systems.
Con Ridiculous package splitting
Even though it makes sense to split devel and the actual binary of an application, the splitting has become as mess in debian and its derivates: for example the nvidia driver is splitted into over 40 different packages.
Con Old software is not useful, but is stable
Certainly for the stable and old stable versions. But the testing version has quite up to date software.
Con Internal quarrels harm the project
Instead of working all together to provide the best Operating System, some maintainers are just ignorant and aggressive to new ideas or new maintainers. That led to many crises and controversy in Debian's history.