Xubuntu uses a customized version of the xfce desktop.
When you install Xubuntu, you get the xfce desktop (customized to look the way the Xubuntu developers wanted it to look), you also get a number of additional applications installed, including:
(1) abiword – a word processor
(2) aisleriot – a collection of solitaire games
(3) bison and flex – tools for developing compilers
(4) catfish – a file search tool
(5) gimp – an image manipulation program similar to PhotoShop
(6) gnumeric – a spreadsheet program
(7) gthumb – an image viewer
(8) leafpad – a text editor
(9) m4 – a command macro processor
(10) orage – an xfce calander application
(11) parole – a lightweight media player
(12) tcl8.5 – a scripting language
(13) thunar – a file manager
(14) xscreensaver – a screensaver
along with some other applications and a number of themes and wallpapers related to xubuntu, etc.
The XFCE desktop will install some applications, including:
(1) orage
(2) thunar
(3) xscreensaver
Clearly Xubuntu Desktop is a lot heavier.
You can download parts of the Xubuntu Desktop seperately. For example, you can install xubuntu-artwork to get the various xubuntu wallpapers and screensavers. I am not sure if you would need to download anything else sinc I haven’t tried it.
If it is of interest to you to have xfce look like xubuntu but to not download the bulk of xubuntu, I can look into it and write an article about it. Of maybe an article about customizing xfce.
You can see a complete list of packages that make up xfce4 here and a complete list of packages that make up xubuntu-desktop here.
Thanks for your question.