Fluxbox is a minimalistic X window manager which supports virtual desktops, highly customized keyboard shortbuts, basic theme management, sorting windows in tabs and .. a menu.
 Elegant and efficient
If you have a clean and organized desktop and want a window manager that manages your windows elegantly and doesn't make a whole lot of fuzz about it, then Fluxbox is for you.
It does the job. There is no eyecandy. There are no fancy icons. There isn't really that much .. to it. It's a clean and elegant window manager.
 But I want to impress hot redhead chicks!
Yes, we all want to impress hot redhead chicks. And they will much likely not get impressed by Fluxbox. The trick that you can play so sucessfully on all those Windows (l)users out there is to run Fluxbox when you are alone and want to get things done and simply switch to Beryl when you want to impress friends and/or hot redheads.
 KDE and GNOME support
KDE / Gnome (Opendesktop, really) compatible programs will dock in Fluxbox's docking area, just as they would make small icons in KDE or Gnome's taskbar.
Fluxbox was based on a the X window manager Blackbox. The Fluxbox developers branched off and created Fluxbox based on Blackbox 0.61.1. It has since improved immensely.
You can right-click your desktop and get a cute little menu where you can configure Fluxbox's settings. This menu can, apparently, also be used to launch applications, and it can be generated using a script which comes with fluxbox that is called fluxbox-generate_menu. However, most Fluxbox users assigns Alt-F2, which "normally" gives a run dialog, to open a terminal such as eterm, aterm or rxvt and mostly just type the application & they'd like to run. (I do remember using Fluxbox's menu to start a browser or something once or twice a few years ago).
 In bullet summary
If you're relatively new to Linux then you may not like Fluxbox because it doesn't have all the bloat - I mean "fancy menus and icons and stuff" - that OS like Windows has.
If you've used Linux for a few months then you'll much likely love Fluxbox.
If you've used Linux for years then you're in love with Fluxbox by now or too used to EvilWM or ION or some other simplistic window-manager to even consider trying it.