The folks at Commoncraft have created a very simple low-tech video on what the heck is a "wiki" and how does it help people collaborate. Wiki's are notoriously known for being helpful but hard to explain. If you are new to wiki's then definitely check it out. If you are pushing wiki's in the enterprise, you may want to forward it.
I'm glad Commoncraft didn't use an example of building a piece of software, but rather used an example about organizing a camping trip with some buddies.
Still, the best way to learn about wiki's... Play with one! You can use PBWiki for free.