DO NOT use this tag, use the more specific Raspbian-Stretch or Raspbian-Jessie.
Raspbian is a GNU/Linux operating system derived from Debian, which has been around for more than two decades. The current stable is 9 (Stretch). Raspbian is the most widely used Pi based distro and the one recommended by the Raspberry Pi Foundation, who distribute their own images of it in several flavors, a full version with PIXEL (a customized graphical desktop environment), and a "lite" version without any GUI pre-installed. However, both versions use the same repository and it is possible to install a GUI on the lite version.
Raspbian runs on all models and includes appropriate kernels for such. The repository provides more than 35,000 binary packages that can be installed using
apt-get and other related Debian style tools.
Beware that while Raspbian packages use the
armhf suffix, they are not the same as Debian's
armhf port, which targets ARMv7 (and so can only be used on the Pi 2 or 3).