Xsession

Once XDM has accepted the password, it runs Xsession as the user. The script loads resources with xrdb, sets up the environment and looks for the user's own ~/.xsession; if there is one it runs, otherwise a default session starts. The file plays the same part .xinitrc plays for startx, with the catch that the session ends when the script does — and with it, you are logged out.

Distributions built on it. Debian's /etc/X11/Xsession runs a directory of small scripts in Xsession.d, where packages can hook in ssh-agent, D-Bus or input methods without touching anything else. The same script redirects all output to ~/.xsession-errors, the file where every program launched from the desktop writes its warnings. It is invaluable when debugging and has a well-earned reputation for occasionally growing until the home directory is full.