The first question that would have to be answered is, "What is a wiki?"
This particular wiki is one that extends the traditional wiki concept in some innovative ways. This very page you're reading is an example of but one of those extensions: This wiki, as with all wikis using Wagn, organizes information into cards, and cards can be nested within other cards.
Unlike many other wikis, this one uses a JavaScript applet called TinyMCE as the in-browser editor for standard Basic cards. Wagn went further and uses a JavaScript routine to save drafts of cards as they're being edited, automatically, every 20 seconds.
There's no special mark-up to learn in order to format cards. Most of the formatting options are in TinyMCE's toolbars, and a small subset of plain old ordinary XHTML can take care of the rest.
There are many more features than can be reasonably fitted here.
If there's anything that should be here but isn't, sign in and add it.
Thanks, and welcome!