The developers of Feng Office call their software a Web Office. There are many other terms that would fit as well: Online Collaboration Platform, Project Management Tool, Groupware or Extranet for example. To put it into one sentence, Feng Office is a Web based software that allows a group of geographically distributed people to collaborate by sharing information over the Internet.
Feng Office is free software (also called open source software). This not only means that you don't have to pay for it, but that you have the right to change the code to fit your needs.
Additional information can be found on the Official Feng Office homepage.