The apps file is responsible for configuring the state of programs when they are opened. For example, you can specify the workspace, the window dimensions and the location on the screen. Also, you can specify programs that should automatically be grouped. Programs are specified using regular expression matching with common X properties.
The keys file allows you to bind specific actions to certain key (and mouse) combinations. For example, you can move or resize windows, change workspaces, open menus, change transparency settings, and much more.
The startup file is one way of starting fluxbox; it is just a shell script, and it is usually run via the `startfluxbox' script. It allows you to start other programs at the same time as fluxbox. I've made some modifications to allow me to start programs after fluxbox starts, and also to restart fluxbox in case it crashes, so I don't lose my X session.
The style is responsible for the appearance of Fluxbox. It controls everything drawn by Fluxbox (and almost nothing that isn't drawn by Fluxbox, such as the contents of windows). This includes the window titlebars, menus, the toolbar, and the slit (dock).
This script allows you to switch workspaces as though they were arranged in a grid. The specifics of it are described here. The intent is that you would run this script using the keys file.
Here are some patches that I've written for fluxbox. I haven't gotten around to fixing these up enough to go into the tree. They're all broken in a current checkout, but you can modify them easily or checkout the revision for which they were made.
This adds a key command that will allow the user to select a window from a menu and then perform a specified action on it, or by default, raise and focus the window. Syntax: <keybinding> :MenuCommand [<shade|maximize|...>];
This adds a resource to the ~/.fluxbox/init file called session.screen<num>.maximizeGeometry . It allows you to provide a standard Xlib geometry string to specify the size and location to which maximized windows should be set. For example, "1020x760-1020-760" specifies that maximized windows will be of size 1020x760 and aligned in the lower right corner of the screen.
This adds some resources to the ~/.fluxbox/init file to configure what happens when you middle-click or double-click on the titlebar of a window. The resources are session.screen<num>.windowMiddleClickAction and session.screen<num>.windowDoubleClickAction . You may specify any command that is valid in the keys file.