This seemly simple question, is not the simplest to answer.
A simplistic answer will talk about the software, and very probably refer to the code github 
. But, that is just the code for a client, server and a bunch of plug-ins.
A more complex answer may talk about the client being a single page application, wikipedia . But, that doesn't really do it, if I wanted a wiki as a single page application, I would very probably using something like TiddlyWiki web .
In reality Federated Wiki is not really about the software, beyond it being an enabler, but rather about a social experiment. The sharing of thoughts and ideas within a community. While it shares the basic idea, from a traditional wiki, that I write about what I know. How others interact with my writing, beyond reading, is radically different.
Rather than simply editing my page directly, as you would in a tradition wiki, you need to take a copy. You can then modify your page to reflect your knowledge, thoughts and ideas.
Then, if your site is in my neighbourhood, I will be also to see that there is a new version of the page. I can then look at it, probably bringing it up alongside my version, and easily see the changes. If I like your changes, I could simply fork the updated page back.