The desktop environment (DE) is what makes up most of the user interface. It may include system panels, docks, window borders, backgrounds, and desktop widgets, among other things. GNU/Linux has a wide variety of DEs avaliable—this page will outline some of the most popular ones.
It is important to note that no DE is tied to a particular GNU/Linux distribution, or vice versa. Thus, if your distribution of choice is Debian, you may use GNOME, or KDE, or both GNOME and KDE (if you installed both). You can install as many different DEs as you want and try them all!
The three main desktop environments are XFCE, KDE, and GNOME.
XFCE is designed to be fast and minimalist
GNOME is simpler to use, and looks nicer. Of course, this comes at the cost of performance (the difference isn't that much though).
KDE is in between GNOME and XFCE in terms of performance and features. If you are unsure, KDE is a good choice.
Tiling windows managers:
Tiling window managers are a lot more difficult to use, and aren't for beginners. Navigation is usually done using the keyboard as opposed to the mouse.