With the command w you can find out who is logged onto the system and what they are doing. Example:

$ w
 15:17:26 up 62 days,  4:33, 14 users,  load average: 0.00, 0.04, 0.01
USER     TTY        LOGIN@   IDLE   JCPU   PCPU WHAT
jj       pts/0     30Mar04  4days  0.50s  0.54s xterm -bg MistyRose1 -e su -l
jj       pts/1     23Mar04  5days  0.20s  0.20s -zsh
jj       pts/2     23Mar04  5days  1.28s  1.28s -zsh
jj       pts/3     23Mar04  3:28m  3.21s  0.50s -zsh
[...]
jj       pts/7     07Apr04  0.00s  9.02s  0.01s w
jj       pts/9     25Mar04  3:24m  7.70s  7.38s mutt
[...]
jj       pts/14    12:49   37:34   0.20s  0.13s ssh totan

The last line tells us that user jj has established a secure shell (ssh) connection to the computer totan.

If any users of other systems have logged in remotely, then the parameter -f will show the computers from which they have established the connection.