Here is the full license as it currently appears in the source code:
"This software is licensed for use in websites which are connected to the public world wide web and which offer unrestricted access worldwide. It may also be freely modified for use on such websites, so long as a link to http://www.question2answer.org/
is displayed on each page."
In other words, you're free to use and modify it for public facing web sites, so long as an attribution is shown, but not (currently) for Intranets and the like. It is possible that this will be loosened in future, or alternatively that there will be a charge for using it internally within organizations. Certainly, I'm happy to give a free time-limited license (say, 1 year) to any organization that wants to use it internally, if they're interested in being part of the beta program.
As for "open source" - the source code is "open", simply because it's written in unencrypted PHP, but strictly speaking this is not a classic "open source" license, because of the limitation above. This is something I deliberated over, and might well change in future, if it appears important to potential users.