A Wiki is a collaboratively edited website. Anyone can add, edit or delete text.
There are many different Wiki "engines" in a variety of programming languages. Different Wikis offer different features, although they all have the same core features of world-editable text. If you want help selecting which wiki engine meets your needs, check out http://www.wikimatrix.org/.
What if someone sabotages a page? - This can and does happen. However, this Wiki, like most, includes revision control. It is possible for any user to roll-back a page to its previous state, undoing the damage. Anyone sabotaging a page can be added to the BannedList by a wiki admin.
What is the point of a Wiki? - It is designed to be a quick and simple way of allowing multiple distributed users to produce a website, and keep it up-to-date. The ease of editing is a priority of a Wiki - most people don't have time to produce a page of HTML, whereas they can probably type a couple of sentences into a Wiki without any problem.
What does Wiki mean? - Wiki derives from the Hawaiian word wikiwiki which means quick.