Noob question: Are there any unique characteristics of the different Linux distros (Arch vs Fedora vs Debian vs Ubuntu)? What makes you use one over the other?
1. package manager. Historically you could make something of a divide between "Red Hat derived distros" that used RPM/Yum (and now DNF) for package management, and the "Debian derived distros" that used apt for package management. Other distros like Gentoo, Slack, etc. were characterized by their own unique approaches to managing packages.
2. Locations of important files. Different distros sometimes vary in terms of where the put "stuff". Is a given thing under /opt, or /usr, or /var, etc? And once you get used to a certain layout, you tend to want to stick with what you already know.
On point (1) above, there was a long period of time when apt was widely held up as being objectively better than rpm/yum, and it probably was. But I think many (most) people would argue that that time is far in the past, and that now the distinction between apt and rpm/yum/dnf is purely a subjective matter of taste. Yeah, a few diehards will argue that point one way or the other, but realistically, either approach is fine.
Debian and Fedora are also characterized by their focus on including only Free software, although their are some distinctions between exactly what either will/will not ship. Ubuntu (a Debian derivative) was famous early on for making it easy to install non-free things, like codecs for playing back mp3 files, etc.
I don't really know much about Arch (never used it), but those are some of the things that define the different threads in the "distro wars".
For the desktop, Ubuntu has a huge thing going for it: proper font rendering configured by default. Every other distro looks like you're back in the 90s, to get decent fonts you're expected to patch libraries or/and fine-tune fontconfig (and enjoy your efforts break or get broken by OS updates). But Ubuntu by default renders text better than OSX or Windows, in my humble opinion. Just make sure to install TrueType fonts from Windows and Mac (Consolas is a must, IMO).
For educational purposes Arch would be my pick because it uses completely different (and vastly simplified) "flat" configuration. Most of the system config is stored in just one file. Also, the Arch documentation, the community and default behavior make it easy to see what's under the hood, yet keep it simple and approachable. If one's goal is to understand how a Linux distro is assembled of various components and how they fit together, Arch is hard to beat.
CentOS/RHEL are great for understanding a typical default server environment used by most enterprises. For example if your goal is to learn SELinux or systemd, that's what I would recommend.
Gnome 3.20 and fedora 24 is simply beautiful. Running it on a XPS 13 has been a brilliant experience.
for what it's worth, i did not find this to be the case when i recently installed Ubuntu 16.04 and had to do a lot of fontconfig tweaking. the major issue being the firefox UI fonts not matching that of the desktop environment
My personal feelings in short form:
Ubuntu - User friendly. Good workstation out the box particularly for media (I use it for an HTPC). Good for people newer to Linux.
Debian - Solidly reliable. Is the base underneath Ubuntu, and as such has a lot less stuff in by default, thereby giving you the flexibility to set it up how you want.
RHEL - Provides good corporate support, costs money.
Fedora - A cutting edge, free, RedHat. I now mostly use this. At work we end up having to use RedHat on some workstations (so makes sense to use Fedora for places where we can't), and some servers need securing with SELinux (which is only really feasibly on the RedHat family).
Arch - Gives you ultimate customisability at the cost of effort (think of it at one end of an axis with Ubuntu at the other and Debian in the middle).
-1) 2010 Toshiba Qosmio
-2) 2013 Chromebook Pixel
-3) 2015 Dell XPS 13 9343
I tried a bunch of different distros and DE's (Ubuntu, Linux Mint, Linux Mint: Debian Edition, ElementaryOS, Fedora, etc.) Everything ran okay on the old Toshiba, but on both the Chromebook Pixel and the Dell XPS--both of which have very Hi-DPI displays--Fedora was the only one I tried on them that mostly worked out of the box vis-a-vis display, keyboard, and trackpad settings. So I've mostly tended to stick with that.
The omission of non-free drivers can sometimes cause problems, however. The Dell laptop uses a proprietary Broadcom wifi driver, so that won't work out-of-the-box in Fedora due to it being non-free. I've debated just tearing out the wifi card and replacing it with an Intel one to avoid dealing with that headache.
Ubuntu is a fairly opinionated distribution built by canonical - it does not necessarily be at the bleeding edge for anything.
I gave just recently switched over from Ubuntu to fedora and have had a much better experience - especially on my latest Skylake Xps laptops with nvme ssd.
Fedora - bleeding edge technologies tend to show up here before any other major distros
Debian - (I have no experience using vanilla debian, so ... n/a?)
Ubuntu - stable LTS releases and broad hardware support if you are willing to put up with/replace canonical's weird unity bullshit and non-oss software.
Debian is pretty much the same as Ubuntu, without all the "weird unity bullshit and non-oss software".
CentOS/RHEL: Aimed at enterprise use, so it's often slow to get package updates (but quick to get security updates), but is very very stable.
I did a test upgrade from 14.04 to 16.04 on a workstation that I didn't care about and it was an unmitigated disaster of broken packages.
Fedora is backed by Red Hat (an American company) while Ubuntu is backed by Canonical (not American).
Fedora is more bleeding edge than Ubuntu. Fedora has SELinux with good policies by default and Ubuntu has App-Armor, but I think disabled by default.
One would use Fedora when developing software for servers (CentOS / RHEL). Ubuntu, however, has more packages in the repositories, so it is more end-user friendly.
Link for ungoogleable name: https://wiki.archlinux.org/index.php/pacman
And this may help to answer your second question: https://www.linux.com/news/best-linux-distros-2016