One of the beauty of Linux is the flavors, yes flavors, so when you look around you find Slackware, RedHat, Debian, SuSe and the new and some how more popular or derived distributions like Fedora, Ubunto, Mandrivia, and some others, so what is the difference between each other.
Let have some history, all those Unix like OS are based on Linus Torvalds original work, and the contribution of million of users and developers around the world, the first distribution was one of the first version of Slackware (first beta release in April of 1993) one of the first Linux flavor the second one in my memory is RedHat (First Linux release 1994), Debian (First release in August of 1993) , from this three distributions came out most of all other distributions we have today, for example Ubuntu is based on Debian, RedHaT has a number of distributions based on it, most related Fedora, Slackware to my surprise was the based distribution for SuSe. To see a list of most distributions and their root look at this
Now days we found lot of support for open source distributions, all support almost the same features, so most of you decide what to use depending of the region you are and the popularity of documentation around. Me still love my Slackware box, how ever to be more related to the commercial knowledge, of the people around me (clients) they know RedHat, and also want to be on the edge of what is new to Linux on the desktop my choice will be Fedora distribution, some people will argue around Ubunto, witch is good but it is easy to explain to some one the configuration of a server on a architecture similar to the one on that server.
Fedora like any other Linux distribution came with two basic graphical environments Gnome and KDE but there are other some include on the distribution you will choose others are ready be added and download (XFCE,sugar,blackbox,etc ). This is one great things on Linux is you can choose a different graphical enviroment, and most of them will vary depending on you machine resources or purpose, but the two most used are KDE and Gnome. To me Gnome is the way to go and some times use KDE on some installations.
Now just to take a pic, Linux graphical environment can be change on every aspect of it if you want some sample look at this blog 10-of-the-best-linux-desktop-customization-screenshots-to-inspire-your-creativity/2008/11/2