In a few words, if someone editing a wiki page writes a "Wiki Name" (consisting of an upper-case letter followed by one or more lower-case letters, another upper-case letter, and zero or more lower-case letters), then if a page of that name exists in the wiki, then a link will appear when the edited page is displayed. Otherwise, a question mark will be displayed next to the name; clicking on the question mark will bring up an editor that will allow the user to add content and save it, thereby creating the page.
All WikiNames in the system are automatically linked in the text.
With the aid of the list of external URLs, it is possible to link to pages on other wikis and web sites. Any reference of the form "ExternalSite:Destination" will automatically link to the appropriate page of the external site. Example: Google:search.
Formatting in wiki follows the principle of WYTIWYG (what you think is what you get), and consists of a pseudovisual markup.
For example, the following:
* test
* test1
is displayed as:
while
1. test
1. test1
is displayed as
Well, what can we say? Double-click anywhere here and start editing this page.
You can even save your changes. So, how does one deal with vandals?
Version control turns out to be an excellent defense against vandals (and is pretty useful on its own, too). Every version of a page is saved in the database and you can view the difference between any two versions
How can all this be used?
“Wiki wiki” is a Hawaiian word meaning “quick.”
The point is this:
You get a wiki site with a minimum set of pages. It won't do anything except serve visitors with that minimum set of pages and give them the ability to edit them and create new pages.
That depends on the settings.
There were no restrictions in the original implementation of the wiki concept: anyone could change anything. This is of limited utility in real life, and wacko provides for flexible management of reading and writing rights.
By default, everyone can create pages.
But you can change the default, either for the entire wiki or on a page-by-page basis.
P.S. You really didn't address the issue of vandals. How do you deal with them?
P.P.S. So it turns out that users have complete control over content?
P.P.P.S. OK, so let's assume you're inundated with vandals and hooligans. The wiki will simply be paralyzed.
1. You won't be able to roll back over “improper” changes, because there'll be so many of them.
2. You won't be able to make “proper” changes, because they'll be lost in the flood of “improper” ones and will end up getting thrown out with the latter.
RomanIvanov: In practice, this hasn't happened. If this happens, I know a number of ways to deal with it. For example, limit access to the content being vandalized only to registered users, or perhaps an even narrower group.
AlexLane: The P.P.P.S. assumes long-term coordination and cooperation among the bad guys, which is hard to sustain for any time. Additional information that addresses this issue is set forth in Wiki:WhyWikiWorks. Plus, vandalism isn't the only thing that can bring a site down to its knees... just getting mentioned in 
Slashdot is enough!