Questions related to logging into a user session or shell, for example at the graphical login screen that normally appears at the end of the boot process
In Ubuntu (desktop version), the user is normally presented with a graphical login screen (called a greeter) at the end of the boot process, where they enter their password. During installation, the user must give a username and password. This password will be used to authenticate tasks that require elevated privileges such as installing software, as well as for logging in. It is possible to select an auto login option, in which case the login screen would not appear unless the user logged out. The graphical login is provided by the display manager, normally LightDM.
If no display manager is installed or no X server can be started, the user may log in on a tty (virtual terminal) instead. These can also be accessed from an X-session, normally by pressing ctrl+alt+F1 (or F2, 3, 4, 5, 6) (switch back to X with ctrl+alt+F7)
When a user logs in to a shell (graphical or otherwise), the shell loads their environment, including local preferences, from configuration files.