Note: if you work on desktop entry files, you should refresh them to see the results: Alt-F2 and run 'r' or 'restart' to restart gnome-shell. Otherwise changes might only work after you log out.
The desktop entry specification creates a standard for application launchers. Gnome adds several extensions to the format which are widely in use, but as far as I can tell undocumented. This is an attempt to document them so I can write my own autostart launchers for gnome. Pull requests are highly welcome.
There is a guide on gnome developer that explains basics about how to integrate an application with the desktop.
Autostart applications run when the user logs into the graphical desktop environment. All desktop managers make custom extensions to the format. This attempts just to cover the Gnome extensions, and won'