CAPTCHA stands for "Completely Automated Public Turing test to tell Computers and Humans Apart". As the name suggests, it is essentially a challenge to users to prove themselves human.
The point of a CAPTCHA is to prevent spambots from performing activities that should be restricted to humans. For example, it can be used to prevent them from creating accounts for email, wikis, etc.
Spambot usage of wikis has grown significantly recently (like all other spambot activities). It is somewhat of a trivial problem, because most wikis restrict editing permissions to registered users, so if one can prevent spambots from creating accounts, that should mostly solve the problem, unless there are super-sophisticated spambots that can actually hack wikis and create pages without having to register, but I don't even want to think about that.
One free CAPTCHA is reCAPTCHA, which can be obtained at recaptcha.net.
Anyway, it might not solve the wiki spam problem completely, but I think it's worth a try! At this moment I'm trying to implement it on my own website, so I'll get back to you if it turns out to be totally easy to use.
Update
Well, I followed the instruction here, http://recaptcha.net/plugins/php/, but I couldn't get it to display, so I guess I really will have to figure out (a) does jones support php? and (b) how to use php, not just paste some code between those little php tags, because when you just paste code, if it doesn't work, you don't know what to change to make it work. Hopefully I will get it figured out because I had prepared quite a treat for you in my form. (Hint: it involves ordering a pretend ice cream sundae to be pretend delivered to a made up address....IF and only if you can prove you're not a CYLON!!)
Aw, looks like I'm outdated. Apparently the hotmail captcha was cracked in february, and the gmail captcha was soon afterwards.
But so I had another idea for keeping spambots from signing up for accounts on this wiki: what if there were just a password mentioned on the front page, where it gives instructions for creating an account, and there's a field for it on the account-creation webpage, so then a person can just go to the mainpage, get the password, and sign up. Since that's probably not a widely-used security technique, I doubt spambots are programmed to deal with it. Security not through obscurity, but through obscure techniques.
Update
I just added an 'are you human' checkbox to the edit page. This is security through variety, or controlled mutation. Since this is the only MoinMoin wiki to use this captcha, it's not worth the spammers' time to write code just for our tiny wiki. I'm going to test the spam prevention for a week or two and then try removing the account creation requirement for editing. (I don't think PHP is on Jones, see JonesLanguages) --NathanSanders