There are three main window managers, or "the way the OS looks and feels", for Linux. Gnome and KDE are the oldest, and XFCE is the newest. There are some advantages to each, and most of the talk over superiority are between Gnome and KDE, so I'll give some more press to XFCE as compensation. So here we go, and please ask questions if you have them.
KDE was the first breakthrough window manager, started in October of 1996. It was based on Trolltech's Qt development library, and some members of the community became afraid that such a crucial piece of software might go proprietary, they split and formed the Gnome windows manager in 1997. Gnome was developed out of the toolkit originally used for the GIMP, GTK+. XFCE was a separate project, started in 1996, that was similar to KDE. It gained the GTK+ toolkit abilities in 1998 and can take install and run programs for KDE and Gnome.
KDE and Gnome both have the advantage of a huge community behind them, as well as the fact that they are the two most prevalent window managers in Linux. KDE typically is considered to have the best programs, and looking through my preferred apps on my computer I would have to agree. Gnome typically has a better user interface and makes an attempt at being lightweight, though in my opinion both KDE and Gnome are overly bloated.
XFCE is rarely talked about, for reasons I will never understand. Try this out, get three 100 MHz computers with 48mb RAM and install Kubuntu, Ubuntu, and Xubuntu on them. Then tell me which one actually runs. XFCE is the lightest of the mainstream window managers, though tiny window manager is still much smaller. It also has the ability to run all KDE and Gnome applications. In my mind, XFCE is the easiest to use for those used to using Windows and can unite the best of all worlds. It's easier to change the look, since any window manager tweaks made for XFCE, Gnome, or KDE can be used. It definitely runs the fastest as well.
If you want to find out which you like the best pick up a live CD and try it out, or if you're feeling adventurous you can always triple boot and see what you think. It's also possible to do concurrent installations and switch at login, but it can be a huge headache. If you have your own opinions, leave some comments.