What is a Wiki? Good question, but not so easy to answer. I'll give it a try, though.
First of all, a Wiki is an idea. The idea of a man called Ward Cunningham who thought that the web ought to be more than TV on steroids: not only a medium that allows everybody to consume information easily but also a medium that allows everybody (and he really meant EVERYBODY) to create and to share information easily (and he really meant EASILY).
In a word, Cunningham realised that for non-technical people publishing on the web is ONE BIG BLOODY MESS. So he wrote a simple piece of software that allowed him and his friends to do just one thing: to share simple web pages without any additional “publishing” software. He effectively traded power for ease of use. And so the first primitive Wiki was born. This happened more than a decade ago and the idea has since then blossomed into a vast universe of Wikis, some as simple and limited as the first, others, like the in(famous) Wikipedia, much more advanced.
By the way, the name Wiki comes from a Hawaiian phrase: wiki-wiki means “quick” or “informal”.
Software on a Web server
Second, a Wiki is a piece of software that runs on a web server and transforms simple text files into web pages (ie HTML) that are then displayed by a user's web browser. In other words: what you see right now certainly looks like HTML, indeed is HTML — but it is not an HTML page stored somewhere on my web server and served to your browser. Instead the HTML you see is based on a text file (like this) that was taken by the Wiki software, dynamically (or on the fly, as the techies love to says) translated into the equivalent HTML and then sent to your browser.
A Wiki also allows to edit these text files through a simple web-based interface. This in turn allows all user(s) not only to view but also to change or create pages at will — from everywhere and without any special software or special knowledge. Internet access and a web browser are enough.
The downside of this extreme flexibility is that Wikis are limited in what they can (easily) do. If something can not be described in terms of simple text, it is not really wiki-able. (Of course, HTML is text as well, but we are talking here about text files with an extremely simple structure.)
Nowadays there are many different implementations of the original Wiki idea around; some are so simple that a six-year old could use them; others are more powerful but also more complicated. I am using a Wiki called OddMuse that's pretty easy to set-up and yet powerful enough so that I can do almost all the things I want to do. And since OddMuse is written in Perl (a very powerful computer language) and since I know a bit of Perl I can always adapt it to my needs and preferences.
Third, a Wiki is often (though not always) a community of like-minded people or those with shared interests. Since editing pages is so simple that even a complete web novice can learn the basics in a couple of minutes, since pages can be edited from almost every computer in the world, Wikis are an ideal medium for collaborative efforts. Have a look at the open source encyclopedia Wikipedia to get a feel for the idea. (Indeed, there is an article on the Wikipedia about Wikis.)
The downside to the wonderful openness of a Wiki is the openness of a Wiki. If the pages of a Wiki can be edited by almost everyone, from almost any computer in the world — then this unfortunately must include the computers of people who want to lure others to their pornography sites, or who want to explain an unsuspecting world why the Nazis were really a rather nice bunch of people or who simply may want to destroy the work of others, either by deleting text in Wiki pages or, slightly smarter, by including more or less visible nonsense on a page.
Most Wikis have several mechanisms to shield against this kind of thing. One possibility is that only known users are allowed to edit pages (that's the case with my Wiki: there is exactly one known user and that's me:-)). That's perfectly fine in my case, as this is just my personal website, but it is of course much too draconian for public Wikis.
These Wikis follow another strategy: they keep around all revisions of all pages for some time (often for a couple of weeks) and they also offer simple ways to rollback a defaced page to any of the previous revisions. This ability is complemented by commands to compare the revisions of pages so any user can easily find out what exactly was changed between two revisions and, if the change is not welcome, initiate a rollback — there and then.
The damage casual evil-doers can do is therefore rather limited: performing a rollback is more a nuisance than anything else. A determined scoundrel can do more damage but for this case a Wiki has means and ways to block access altogether, either for a specific user or whole groups of users. (A really determined person can always go to the place where the server hardware is located and try to physically destroy the server: at some point adding more and more software security does not make the whole system much safer — after all there are always other doors for an intruder. Real doors, for instance.)
Still, vandalism is a problem for almost all Wikis; however, an active Wiki with many users who regularly visit is not too vulnerable: a rollback is quick and easy, and bona-fide users have an active interest in keeping “their” Wiki clean and tidy. In fact, the reason why Wikis work after all lies probably in the simple fact that the average user of a Wiki is much more determined to keep a (for him) valuable resource accessible and clean than the average cracker is to destroy it.
In any case, the advantages (the ease of sharing information, access for everybody, the enormous flexibility) definitely outweigh the possible problems. If it were otherwise, Wikis would not have been such a resounding success.
$updated from: Wiki.htxt Sat 18 Jan 2014 13:14:24 thomasl (By Thomas Lauer)$