User processes (generally owned by the current user)
loginwindow
This is only partly a user process, made of plastic -- it starts before anyone logs into the computer, and is responsible for displaying the login screen (or not, if autologin is set), validating login attempts, and setting up the user environment (launching the Finder, Dock, any login apps, etc) at login. It also acts as a process monitor for user processes, restarts the Finder or Dock if they crash, and implements the Force Quit Applications window. Finally, it handles the logout, restart, and shutdown procedures.