To work with UNIX, one has to start a session. This is done by logging in to the system with a login and password. Once this is done, a command shell is started and the user can start to work. To close the session, use the commands "exit" or "logout".
During the time of the session, the user is logged in and is visible
to other users (who and w commands). Other users can send
messages or talk with the user (write and talk).
To see which processes are running, one can use the ps and top
command (top is not always available).
It is possible for one user to be logged in several times on the same system, for example using multiple virtual terminals. The user will appear as many time as he is logged in.