Captchas are things that are easy for humans to do but hard for machines to do (I wrote a post about this for that google image matching thing: [Harnessing People’s Boredom]).
The most common usage are those numbers and letters that are a bit squiggly which you have to type in to add a comment to a blog. They are obviously there to stop spammers from adding spam to your blog. A lot of very serious technology people have worked on making these reasonably easy for you to read but impossible for the spam software.
But the other place you see a captcha is when you get spam these days. A lot of spam at the moment have those pictures in them which say that you should by such and such a stock. And the way they get round your spam filter is that they are using a captcha to stop the spam filter from recognising the words in the image that they are sending you.
Talk about ironic.*
*But is it ironic though? I say YES!