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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 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 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 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 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 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 Systemd is everywhere
Systemd is very hard to debug by itself, and can break in very strange ways (such as not being able to mount a single partition on bootup), but the increasing number of desktops that are relying on specific functionality provided only by systemd (policy kit, dbus, etc) makes the entire system more fragile and harder to debug.
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.
Recommendations
Comments
Flagged Pros + Cons
Con No default support for unfree drivers
Debian does not official support unfree software so if you have a wifi card or anything elese that requires an unfree driver you will need to download the unofficial/non-advertised non-free iso image which contains all unfree driver packages.