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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.
SpecsUpdate
Pros
Pro 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 Comes with over 55.500 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 55.500 packages (software that is precompiled and ready to be installed on a local machine) -- all of them for free.
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.
Cons
Con No choices
Debian basically doesn't give you any choices, even dpkg supports this feature with virtual packages: for example, it is very hard to use a different sound system than ALSA, like OSS4 or to use ALSA with pulse audio emulation, and the same goes for different init systems.