RE:Captcha like captcha only better! It's accessible to the partially sited and blind. It uses a web service from recaptcha.net, so it is constantly being updated to fight spammers without you needing to do a thing. Furthermore you will be helping a good work too! The Captcha uses images of text where the OCR has failed during the digitization of old books. By solving these tests you also help correct the conversion errors and make old works available to a new generation.
Once you've obtained the keys and set the plugin options, there is nothing else to do. It will insert the image tests and work.
The options are self explanitory. Sign up for the reCAPTCHA and Mail Hide keys at http://www.google.com/recaptcha/whyrecaptcha and enter them into the appropriate fields in the plugin options.