Debian is arguably the most important Linux distribution. From it springs such popular Linux distributions as Mint and Ubuntu. Outside Linux's inner circles, it's not that well known because it's purely a community operating system. There is no company behind it, as there is with Red Hat and CentOS, Fedora, and Red Hat Enterprise Linux (RHEL). Without fanfare, Debian is more than just the foundation for other better known Linux distros, it is a powerful desktop and server Linux in its own right.
Debian, which dates back to 1993 and the efforts of Ian Murdock, is now up to date. Nowhere does that show more than with its controversial move to the systemd system and service manager.
While systemd works well in my experience so far with Debian 8, a group of Debian developers, who call themselves the "Veteran Unix Admins (VUA), have forked Debian. They did this because systemd is both now Debian's default init system and Debian allowed software packages to depend specifically on systemd. While that means that programs needing systemd will run on Jesse, such as some GNOME 3.16 programs, some programmers feel this forces them into an overwhelming web of systemd dependencies.
VUA's solution has to been to fork Debian. Their distribution, Devuan, pronounced "DevOne," is still a work-in-progress. At this time, Devuan is only available as source code. The group's goal is to make a systemd-free Debian fork available soon. With it, "users will be able to switch from Debian 7 to Devuan 1 smoothly."
In the meantime, Debian comes with greatly improved Unified Extensible Firmware Interface (UEFI) support. This means it will be easier than ever to install Debian on Windows 8.x PCs with secure boot.
The new Debian also includes pretty-much every Linux program out there either in its DVD images or its repositories. If you want to dig deep into new programs, or the source code, Debian makes it simpler than ever by proving a browseable view of all Debian-related source code. To make it more manageable, there's also Debian Code Search, a source-code search engine.
The operating system also includes a wide-variety of updated programs. Debian, however, is not a distribution you should look to for the most bleeding edge software. For that, you should use Fedora.
One thing, which may puzzle new users, is that Debian doesn't appear to have the Firefox web browser or Thunderbird e-mail client. These mainstays of many Linux desktops actually are present, but they've been slightly forked. For Firefox, Debian uses Iceweasel for Firefox and Icedove for Thunderbird respectively. Under the name, they're the same programs. The name changes are the result of a 2006 dispute between Debian and Mozilla. For all practical purposes, the programs are the same.
Unlike previous versions of Debian, you may now choose to install some proprietary programs. Still, if you're really serious about free software and you refer to Linux as GNU/Linux, it's still the distribution for you.
What Debian isn't, is a distribution for new comers. If you're new to Linux, I'd recommend Ubuntu or Mint. Both are based on Debian, but come with a lot more hand-holding. For power users, or businesses that want a very stable operating system and have the in-house expertise to manage Debian, Jessie is well worth your time.