If you'd like to find out who has been on when and how long, open a console (xterm) (or type at the cli if you're already there):
Depending on the usage of your box, you might get quite a long list. However, normally last
is reset according to a schedule (in order not to let the log file grow to big), so you'll only see the logins since the latest reset.
To view the output page by page, use
For anything else, consult man last