LXSession is the default X11 session manager of LXDE.
(LXDE: Lightweight X11 Desktop Environment)
http://lxde.sourceforge.net/

What's LXSession and who needs this?
A session manager is used to automatically start a set of applications and
set up a working desktop environment. Moreover, when the user logout, it 
remembers the applications in use, and restart them the next time you login.

Gnome is bundled with its own gnome-session, KDE brings its own session 
manager, too. XFCE and ROX desktop also have their own session managers,
xfce4-session and rox-session.

The main difference between LXSession and the preceding programs is that 
LXSession is lightweiht, and it's not tighted to "any" desktop environment.
It's desktop-independent and can be used with any window manager.
With proper configuration, you can make your own desktop environment with
LXSession. This is very useful to the users and developers of non-mainstream
window managers and desktop environemts.

Here we use our own desktop environment LXDE as a working example 
to tell you, step by step, how to create your own new desktop environment.

Create a startup script for your desktop, and put it to /usr/bin.
For example, we create a script '/usr/bin/startlxde'.

Then, add the commands you want to execute *before* LXSession, 
such as setting up locales or others, and put "exec lxsession" in the last line.

For example, our startlxde script looks like this:

#!/bin/sh
exec /usr/bin/lxsession -s LXDE

Apparently, LXDE is the name of our desktop.
Replace it with the name of your desktop.

Then, make a desktop entry file for it under '/usr/share/xsessions'.
With this, you can select this desktop session from menu in GDM.
For example, this is the content of our LXDE.desktop:

[Desktop Entry]
Encoding=UTF-8
Name=LXDE
Comment=LXDE - Lightweight X11 desktop environment
Exec=/usr/bin/startlxde
Type=Application

Apparantly, you can replace the name and description with your own.
Exec should points to the startup script created in previous step.

Now you get an item 'LXDE' in the list of available sessions in gdm.
NOTE: Restart of gdm might be needed. ( On Debian:  sudo /etc/init.d/gdm restart )

So, what programs will be execuated when LXSession starts?
List the programs you want to execute in this file:
/etc/xdg/lxsession/<Name of Your Desktop>/default

For example, our /etc/xdg/lxsession/LXDE/default looks like this:
smproxy
openbox
lxpanel

smproxy is a program provided in xorg. It provides session management
support for programs which don't know X11 R6 session management protocol.
So it's highly recommended you include this line in your desktop.

openbox is the window manager we use, replace it with your window manager.

lxpanel is our desktop panel, you can replace it with anything you like, 
such as avant window manager or others.

List other applications you want to launch, one command per line.

Note that there is no '&' at the end of every line since this is not a
shell script.  Don't append & at the end of any line.

OK, it's time to login to your desktop environment. Please try it out.
Wait!  How to logout from LXSession?

Simply executing this command:
lxsession-logout

This will give you a good-looking logoug dialog.
If gdm is installed, lxsession can do shutdown/reboot/suspend via gdm.
(These options are not available if gdm is not running.)

If you want to customize this logout box further, try this:
lxsession-logout --prompt "Your custom message" --banner "Your logo" \
--side "left | top | right | bottom (The position of the logo)"

We create a script '/usr/bin/lxde-logut' to do this:
#!/bin/sh
/usr/bin/lxsession-logout --banner "/usr/share/lxde/images/logout-banner.png" --side top

You can put this logout script in the menu of your window manager or desktop panel.
Then, you can logout via clicking from the menu.

Have fun!