w

The w command prints out who's logged on and what they're doing.

USER     TTY      FROM              LOGIN@   IDLE   JCPU   PCPU  WHAT
cyan     pts/37   195.218.110.118  12:08    0.00s  0.15s  0.04s  w

From this you can see where I'm logged on from, what time I logged on, how long it has been since the system detected a keystroke, and what it is I'm doing (typing w, as you can see).

You can customise the output of w, for more information see the man page.

If you want to see the last few logins to the system, you can use the last command.