Technically they are the same "Linux" which is just the kernel... Also, it is not really Linux, it is GNU/Linux, as the compiler that Linus originally used was released by the GNU foundation... They might or might not contain different kernel versions... 2.6.25-R5, or 2.6.24.19 etc... but that is not what makes the different "flavors" or Distros of linux different.
In reality the different distros (Distributions) vary the most by what packages they install by default, what packages are available using their "package manager" and the actual package manager itself.
Generally, major "branches" of linux are differentiated by what package manager they use... For example, ubuntu, kubuntu, edubuntu, xubuntu etc. are based on Debian and use debian style disto packages. Redhad/fedora uses RPM (Redhat Package Manager) style packages. Slackware uses tgz files, Gentoo uses portage for package management which is very similar to BSD's ports package management system. Gentoo essentially compiles every program from source code, so it is optimized for the precise CPU and other hardware on your system. It's a very elegant solution, but since you are not downloading pre-compiles packages and just "unzipping" them into the right places, installing packages and updates is much slower in Gentoo...
Unix is a closed-source operating system that is patented. It is NOT Linux... Linux was "patterned" after Unix, but does not contain any proprietary Unix code. Source code is pretty portable between Unix and Linux, but not the compiled packages.
Different distros have different "packages" that are available pre-compiled for the system. Ubuntu has one of the largest repositories of programs available for it using the package manager, (With the possible exception of Gentoo.) but as with ANY linux distro, you always have the option to download the source code, and compile it... usually by un-tarring (unzipping) the source code, going into the directory and issuing the ./configure && make && make install command.
The two most popular desktop envirometns are KDE and Gnome. The look and feel of the two are different, with KDE usually having a definite edge in usability, although it also takes a bit more system resources than Gnome. A disto will generally have one desktop that is preferred over the other (installed by default) ubuntu uses the Gnome desktop, kubuntu uses the KDE (K Desktop Enviroment).
Certain programs are dependent on certain libraries provided by one or the other desktop (For example, probably the best music player, bar non, Amarok, uses certain libraries provided by the KDE desktop, while Evolution, an excellent email client, requires the base Gnome libraries. The good thing, however, is that regardless of which desktop you decide to use (and there are dozens of others besides Gnome and KDE) you can run both Evolution AND Amarok on the same desktop. If you use KDE and install Evolution, the package manager will install the needed base libraries for Evolution to work, the reverse is also true if you decide you want to run Amarok under the Gnome desktop, or XFCE desktop (the default in xubuntu... XFCE is a very light-weight (fast and uses little memory) desktop that is quite usable, but not nearly as feature rich as KDE or Gnome.
I hope I haven't confuse you too much... In reality it doesn't matter all that much which distro you pick, as long as it is decently supported with security patches and updates. Gentoo will always be the most "up to date" of any linux system because if you tell it to use the "cutting edge" packages and you do:
emerge --sync
emerge -uDNav world
your system is *completely* current... there is never an "upgrade from version 10.1 to 10.5"... if you installed gentoo from 5 years ago and did those two commans above, it would be exactly the same as a system you installed from the 2008 version of the CD... (It might, however, take 2 days for the upgrade to complete, after re-compiling every program on the system after 5 years of updates...) (Gentoo is also not really what I would consider a "beginners" linux... There are occasional package conflits and it comes much less "pre-configure" for many programs. It is that way by design.... allowing YOU, the USER to decide for yourself what features you want compiled into each program... For example Amarok will not contain the code for using MySql for the database to store your history, titles, album covers, lyrics etc... You would need to specify that you wanted that support... but the advantage is that the system is smaller, more compact and faster without all "excess" code compiled in. In most systems, you would get the code for Mysql, Postgresql, sqlite, and 2 or 3 other databases all compiled into the program, you don't have the choice... it's just there if you want to use it.
Redhat, slackware, etc.. have a very long, slow release cycle... they are usually running older versions of most of the components... Theoretically more stable/tested versions, but also usually less featured and often slower...
I hope I haven't confused you with too much information.