Linux: How does user log out work

I'm new with linux. I've searched about how to safely log out other user and found few solution but I want to know how they work inteally. I have few questions:
1. Does logout only kill gnome-session and every its child process and nothing else?
2. pkill -KILL -u {useame} Does it just send SIGKILL to every child of gnome-session? and does it ensure that every child process is safely killed?