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

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.
Besides, LXSession has a built-in lightweight Xsettings daemon, which can 
configure gtk+ themes, keyboard, and mouse for you on session startup.
In gnome the Xsettings part is provided by gnome-settings-daemon.

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.

LXSession can start a set of programs specified by the distribution
makers or users. Furthermore, LXSession is more advanced than
some of the other ones because it can ``guard'' the specified
programs, and get them restarted if crashes happened.

Besides, the major difference between LXSession and the preceding programs
is that LXSession is lightweight, 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 environments.

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

Apparently, 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 executed when LXSession starts?
The window manager, of course, should be the most important one.
It should be specified in this file:
/etc/xdg/lxsession/<Name of Your Desktop>/desktop.conf
The file is a plain ini file, like the desktop.conf.example
bundled in this package.

Then, list the other programs you want to execute in this file:
/etc/xdg/lxsession/<Name of Your Desktop>/autostart

For example, our /etc/xdg/lxsession/LXDE/default looks like this:
@lxde-settings
@lxpanel -p LXDE
@pcmanfm -d

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.
Since this is not a shell script, shell expansion like ~ or $HOME won't
work. If you need this, write a script and do the shell things there,
than list the script here in the autostart file.

If a line starts with @, the program will be guarded by lxsession.
That means: If the program crashes, it should be automatically restarted.
If the program is terminated normally, or is terminated because it
receives SIGTERM or SIGKILL, then the reload won't happen.

The default system-wide config files under /etc/xdg/lxsession, can be
overriden by user-specific ones. If the user specific config files are
found in ~/.config/lxsession, the ones in /etc/xdg/lxsession will be overriden.
So this leave some flexibility for the users.

Another way to start programs on session startup is to add their *.desktop
files in ~/.config/autostart. Please refer to the Autostart spec.
http://www.freedesktop.org/wiki/Specifications/autostart-spec

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 logout 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!