You'll notice a change when you try to leave a comment on this site. I have enabled the "Word Verification" option. This just means that you will see an image of some letters that you need to type in order to leave a comment.
I have recently started to get a number of "Spam" comments in the form of: "Love your site, Check out my site on stamp-collecting..." etc. These spam comments are generally created automatically on blogs that allow anonymous comments. As the programs that generate these messages have not yet learned to read, but most people who would want to leave a legitimate comment have, the word verification thing is a nice way of still allowing anyone to comment, but keeping the spammers at bay.
Feel free to have a go... but really... I'm not interested in stamp-collecting! ;-)
[My thanks to the mysterious QuakerOatsGuy for pointing this option out]