4.) The XFce panel
The most visible part of XFce is the panel. Here's where the application launchers, pager and all kinds of plugins reside. There's no single "Start Menu", so this may take some getting used to, especially if you're from the KDE/GNOME stable, or worse, a Windows refugee!
The buttons with the menus popping out of them are called, obviously, panel menus! The button itself launches one command from the menu, usually the most common of the lot. You can also have buttons without the menu attached. Just right-click on the button, and uncheck the "Attach menu to launcher" option.
On the panel, these menus are typically created category-wise. For example, you could have an "Internet" category, with the button itself launching Mozilla, and other entries being Firefox, Evolution, GAIM, among others. Another button could be the Multimedia category, with the default being XMMS and other entries being MPlayer, Rhythmbox and Totem. You'll figure out pretty quickly how to manipulate/customise these panel menus.
These menu settings are an XML-like file in /etc/XFce4/XFce4rc (for default settings) or in ~/.XFce4/XFce4rc. Hack away at will to auto-generate these menus, if you like!
Now for the plugins - these are what give XFce its power, and there are plenty of them! A lot of them have counterparts in KDE and GNOME. Describing them all here would be repeating the information on the XFce website. Point your browser to the XFce overview page, and navigate to the bottom of the page. They've put up a few screenshots of the plugins.
Here is a screenshot of my panel:
The leftmost three icons are panel menus. From left to right, they are - xterm (no menu attached), Internet (Firefox default app) and Multimedia (XMMS default app).
To the right of the separator, then, is the Notes plugin. These are sticky yellow windows to type in simple text notes - exactly like Post-It notes that pile up in dozens on most refrigerators!
After this is the graphical pager. The best part is, I can drag a window previewed in the pager (for instance, the GVim window in the leftmost workspace) in the picture above, to another workspace, and it'll actually move! The only other place where I've seen this kind of functionality is in Enlightenment.
The plugin after that is "Show/Hide All Windows", followed up by buttons for the Settings Manager, Logout and another panel menu I made for halt/reboot.
The green bar is the Volume control. You can increase/decrease the volume with your mouse's scroll wheel (another reason why I'm a huge fan of scroller mice!). The orange/yellow bars monitor network traffic - In/Out respectively. The Blue bar monitors System Load and displays the current uptime. This same plugin also monitors, if you enable it, memory and swap utilisation. To their right is a mini command-line, and the Date-and-Time plugin.
5.) Taskbar Replacement
Personally, I find the XFce taskbar too limiting. You can't right-click on a taskbar entry and choose "Close" or "Move to Workspace X". You can't add plugins to the taskbar. All you can do is change its height! Why not use the GNOME panel instead?
I first saw this being implemented in a few screenshots on themedepot.org. The idea is simple - you don't need to be running a GNOME session to run the panel (just like you don't need to run a KDE session to use kicker, the KDE panel). I chose gnome-panel over kicker because GNOME uses the GTK2 libraries, just like XFce, so won't need to load its own graphical toolkit libraries. Besides, the panel will use the same GTK2 theme I use for XFce, so it'll blend in really well.
Using gnome-panel instead of the XFce taskbar is easy. In your local XFce startup script - ~/.XFce4/xinitrc, comment out the line which launches the taskbar, and add the line "gnome-panel &", so that the relevant part looks like:
Now, on my gnome-panel, I not only have a more functional takbar, but I've moved the clock onto the panel, along with the system tray. I also have a gnome start menu in the corner:
Neat! Ah, the sheer flexibility of UNIX!
6.) The Desktop and root menu
By default, right-clicking on the desktop brings up a 'Desktop Menu'. The structure of this menu is defined in another XML-like file: /etc/XFce4/menu.xml. This file is extensively commented, so modifying it is very easy. To personalise your desktop menu, copy this file into your ~/.XFce4/ directory, and have fun!
A middle-click on the desktop brings up a list of all opened windows ordered by workspace. It's also a quick way to add or delete a workspace.
Scrolling on the desktop cycles through workspaces! (So does scrolling on the graphical pager in the panel). This is by far the easiest way of moving between workspaces (I have between 4 and 8 workspaces, depending on what kind of projects I'm working on). Now I'm so addicted to this, I can't do without it.
For those of you who like lots of icons and documents on yout desktop, XFce might be a disappointment. By itself, XFce has no support for desktop icons. There are workarounds, however, in the form of dfm and iDesk.