The thing about the different flavors of Linux is because Linux itself isn't an operating system. It is a term used to reference Operating Systems using the Linux Kernel. In Microsoft terms. You have the kernel winnt, it is what provides you with the pretty blue-screens. The operating system is Windows Vista, Windows 7, etc. Since the Linux kernel is free and open source, you see a lot of different operating systems built around it. They all do something different and provide a different subset of features.
Ubuntu is by far the most popular up-and-coming Distribution. It has a company behind providing support and development. For most users this is the most complete one out there. I think some others out there that I see a lot of good support for are Mandriva, Suse and Fedora. Fedora was started by Red Hat, and it is more up-to-date then the RedHat Linux distro (which is meant for servers). Suse is made by Novell, which struck a deal with Microsoft for patent protection, whether this hurt and helped Suse, I can't really say. But it's latest release has gotten some really good reviews. Mandriva is another one with no really big company behind it, but like the others, provides a great desktop experience.
The differences in the distributions is mainly how the Operating System handles the configuration of itself, and the package management. Package management is something people outside of Windows don't grasp the first time around, but basically all modern distributions use their own means of installing software, which makes it very easy to update software and stay current.
Desktop Enviroments / Window Manager
Most modern - user friendly - distributions are based on Gnome or KDE. These are your Desktop Environments. The provides a File Manager, Window Management and Desktop capabilities. File Manager is synonymous with Windows Explorer. Window Management is the Title bar that lets you minimize and maximize, task-bar , system tray, etc. And the Desktop is the , well Desktop. It controls your wallpaper and icons and widgets that sit on it.
From here, most distributions are the same. The provide pretty much the same base packages.
If I had to recommend one one of the easier to use ones I would say go with Ubuntu or Suse. Both allow you to run Gnome or KDE. Right now, I have just switched from Gnome to KDE, and I really like it. That is just a matter of preference. While it can be helpful to figure out which you like up front, you can switch to either using the package management of your distribution.
If you are looking to get a better understanding of Linux in general, then Ubuntu and Suse will barely get your feet wet, but are great for those that just want a free Operating System to use e-mail and web. To get a better understanding of Linux I would really look at using a sourced based distribution. My favorite (use every day) is Gentoo. The set up time takes a lot longer b/c you are compiling all the software as you go. While you might go through several installs of Gentoo in the first few months, the knowledge you gain can be invaluable.
In the end, there is no right or wrong answer. You will eventually find one that first perfect with what you are wanting and with how much you want to work with the system versus just using it