Linux is extremely flexible. Linux makes it possible to run in a number of different desktop environments. The plus side of this is that you decide how you want to work. Your system works the way you want it to and not the other way around. The downside is exactly the same. Let's face it, being told what to do is often easier, even if it means getting used to working in a way that you may not particularly like at first not necessarily better, but easier.
On that note, at some time when you've gotten comfortable with your Linux system running the KDE desktop, I'm going to ask you to be brave and experiment with some of these other environments, whether it be GNOME, WindowMaker, IceWM, or one of the many other desktop environments available to the Linux user. You may find yourself totally taken with a different way of doing things. All your programs will still work as they did, but the feel of your desktop the experience, if you prefer will be all yours. For now, we'll stick to KDE.
KDE is the most popular desktop environment in the Linux world, and deservedly so. It is beautiful, slick, mature, powerful, and easy to use. It is also loaded with great applications for e-mail, surfing the Web, playing movies, burning CDs, writing documents, working with spreadsheets, and so on. KDE also features a great collection of games that should keep you busy for some time.
A Few Words about X
In a few seconds, when I start showing you around your desktop, what I am telling you now will fade into the background of your memory, but I still think you should know. KDE, that great-looking desktop system, is the friendly face that rides above your Linux system's real graphical engine. That engine is called the X window system, X.Org, or simply X. What KDE, your desktop environment, does is provide control of windows, borders, decorations, colors, icons, and so on.
When you installed your system, you went through a graphical desktop configuration step of some kind. What you were setting up at that time wasn't KDE or GNOME, but X. X is what the desktop and every graphical program you run really runs on.