The Beginner's Guide to Linux, Part 3: Choosing Your Window Manager and Desktop Environment
GNOME uses Nautilus as its file manager. This program is a cross between Mac OS X's Finder and the version of Windows Explorer used in Windows Vista. Nautilus's main strength and weakness is its simplicity; although Nautilus is easy to use, it also omits tools that are found in much more powerful applications like Konqueror. Nautilus also supports remote connections (SSH, FTP, etc) through a “Connect to Server” dialog that is very similar to OS X.

If you appreciate simple, uncluttered interfaces or come from a Mac OS X background, you should give GNOME a try.
The Xfce desktop environment is very similar to GNOME, but has much less demanding system requirements. (recent versions of Xfce are based on GTK just like GNOME is) Due to this similarity, we chose to classify Xfce as a subset of GNOME rather than give it a section of its own.
Xfce's applications greatly resemble GNOME, but are designed for greater efficiency. For instance, Xfce's Thunar file manager looks like a trimmed-down version of Nautilus. If you like GNOME but have an older computer that can't run it very well, you should definitely consider Xfce.
KDE 3.5
KDE (K Desktop Environment) is as widely distributed and popular as GNOME. As a true desktop environment, it caters to power users with a wide selection of robust software built using the QT toolkit. (the previously non-free component mentioned in the previous section) KDE 3.5 was the last version of “classic” KDE. KDE 3.5 is the one of the most Windows-like desktop environments in common use today, since most programs are accessed from a “K” menu in the same position as the Windows Start menu. In addition to this, KDE 3.5 has a large control panel application that many Windows users will feel at home with. (in contrast, GNOME's system administration tools are more decentralized and exist as separate “modules” on the administration menu)

Although KDE 4 is presently being adopted by more and more people, KDE 3.5 is far from being obsolete. It is still a fully mature and well-tested desktop environment ideal for production use and will remain so as long as security patches continue to be released. While GNOME emphasizes simplicity, KDE 3.5 emphasizes options; any program written for KDE likely has more options and features in it than you can shake a stick at. While many users praise this design for allowing them to know exactly what they have at their disposal, others claim that KDE programs are overcomplicated and appear cluttered as a result. As with the GNOME simplicity debate, the KDE clutter debate is largely a matter of opinion.

KDE's real power comes from the amazingly versatile Konqueror program. The misspelling is deliberate and correct in this instance; versions of KDE prior to 4.x had a trend of including a k in the name of an application or proceeding the name with a k (e.g. kontact or kwrite) to indicate an association with KDE. This naming convention has since been abandoned in KDE4, but some existing programs still use it.

Konqueror (a play on Internet Explorer) doubles as an excellent browser (using KHTML, a relative of the Webkit engine) and a very full-featured file manager with more versatility than Nautilus. Konqueror's file manager supports FTP/Graphical SSH in addition to local file management and features split views, tabs, and close integration with the terminal. Likewise, the web browser component of Konqueror supports tabbed browsing and very good standards compliance, even though it is not as customizable as Firefox.
However, KDE 3.5 has a few problems. Late in KDE 3.5's development, the KDE team decided to split web browsing and file management into two separate applications: Konqueror (now officially classed as a web browser) and Dolphin. (the new file manager) Many users criticize the KDE 3.5 version of Dolphin for lacking essential features like directory trees and consider the KDE team's decision to replace Konqueror with it as a mistake. Fortunately, Konqueror still has its file management capability, so it is easy to keep using Konqueror for this purpose instead of Dolphin. Many of the applications also tend to flicker slightly at certain times (particularly when you scroll) but this is not very noticeable unless you are specifically looking for it or are overly sensitive to things like that.
If you like using a well-proven desktop environment that is somewhat similar to Windows and comes loaded with options, you might feel at home with KDE 3.5.