Almost all distros are based on the linux kernel and that's why they're usually called Linux. However, that doesn't mean they use the same *version* or, even within the same release, it doesn't mean they haven't done changes themselves to the sources (Ubuntu is known to do that).

The difference doesn't only come from the kernel though. In fact, the only difference you'll notice between different kernel versions is drivers supports ( usually,of course there are other changes) and the key difference between distros is the other software that comes with them and how they're compiled/configured.

And about the desktop environments: try them out and see what's the one you like the most (I personally like KDE, specially with the latest releases :) )

Regards

To the some extent, no of Linux distros may hinder adoption of that. There r so many and so many, in which names we do not know / never use. But, though Linux is open source, so anyone can make his own Linux distro with maintaining international standard. The main points r : there are too many distros, the various Linux desktop products r not polished enough (not bugs free / there r lots of probs regarding HW), consistency / usability issues exist with the various Linux distros and last but not least the various suppliers of Linux distributions focus too much on heavy duty tech-savvy users (like admins), and it is difficult to operate for normal everyday users.
Windows is a proprietary OS, so its technology is kept by some of fistful people, though it takes basic techs from Unix / Mac.... Now, u observe, Wind7 also taken concept from Linux specially on security related. Linux is now growing, people now understands what super techs it has and it is not enough to learn Linux /Unix very easily. Though some famous modern Linux (viz. SuSE, Red Hat, Fedora, Ubuntu, Debian) has the same techs that a very simple user can do their preliminary works easily. But in respect of Server admin, there r no of techs that already hidden and the Linux admins try best to discover cost-effective solutions from Linux Os, while different admins may use diffs flavors. It is not enough to complete certs on Linux OS (viz, RHCE, NCLE, LPIC, CompTia etc. etc.) a hands on experience is very necessary for Linux OS. Linux market is grown and it is increasing while data collected and it shown the market share of Linux jump from 0.96% in January 2011 to 1.41% by the year’s end.

"I thought that Linux distributions were based on the same Linux kernel, so why would one distro not be compatible with another?", well, although Kernel is same, but have u checked if the distro uses latest vers of kernel. Already mentioned that, command line almost same, but basically diffs r on GUI based. Red Hat / SuSE supports rpm / yum based, while Ubuntu / Debian support .deb package.... SuSE gui based is yast while Ubuntu based on apt-get / synaptic package manager. It is not mean that SuSE never supports .deb package, but there is some mechanism which can support other flavors package / commands.

"I just wanted to mention that every time I type "distro," autocorrect makes it "distress." Do you think my MAC is trying to tell me something?", friend, there is no english word like distro, that is why it makes sence like distress, this is my guess....