So what's in a wiki?

I particularly liked the- Library Success: a best practices wiki, and- Book Lovers Wiki (Princeton Public Library).It would be useful if TAFE teachers could enter reviews of (for example) DVDs and other favourite titles - I think their fellow teachers would find this useful, and it just could encourage them to log on to the library site more.
Because the information can be changed so easily, I still haven't decided how best to use one, who would use it, etc - need to think a bit more on this one!