When I first started using Linux, I was fond of invoking the 'who' and 'w' commands to find out information about users who are logged in the server. Sometimes I'll send them a message across the network or issue a 'talk' or 'ytalk' request. These was pre-Instant Messaging. :) Anyhow, now you can use an enhanced tool that not only lists who are logged in but also displays what applications they are running. To do this, you need to install 'whowatch' by doing
Running it will give you a screen similar to the one below.
And just an example of what other info you can get, see the next screenshot.
You can play with the other options to find out more about what your users are doing when they're online. Yeah, I know this is also called. Hahaha. However, there are rules that each system administrator follow, unless s/he is the BOFH.