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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.
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Pros
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 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 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.
Cons
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: https://packages.debian.org/en/source/sid/nvidia-graphics-drivers