finger

The finger command prints out the information the system has about a user.

$ finger [username]

Finger will tell you the name of the user you're interested in, when they last logged on, and from where. The [username] part is optional. Looking at some examples:

$ finger cyan
# find out information about the user cyan

$ finger
# will print out a summary of the information about everyone who's logged on

Also some people have .plan files in their home directories. The contents of the .plan file are included in the finger information. For more information check out:

# the manual page for finger
$ man finger

# the manual page for chfn [changes the information finger prints about you]
$ man chfn

Finger will tell you lots about one user, or a little bit about everyone. If you want to know more (such as what someone is doing right now), you need the w command.