Mollom is an "intelligent" content moderation web service. By monitoring content activity on all sites in the Mollom network, Mollom is in a unique position to determine if a post is potentially spam; not only based on the posted content, but also on the past activity and reputation of the poster. In short, Mollom handles incoming posts intelligently, in much the same way a human moderator decides what posts are acceptable. Therefore, Mollom enables you to allow anonymous users to post comments and other content on your site.

How it works
Mollom may block a post outright if it is from a known spammer. If Mollom is unsure how to classify a post, it may require the completion of a CAPTCHA to accept the post. Posts that do not match a "spammy" text pattern and do not originate from known spammers are accepted without the need to complete a CAPTCHA. Essentially, Mollom acts as a proactive content moderator that is on the job 24 hours a day, 7 days a week.
A stand-alone CAPTCHA solution, which neither considers user behavior nor point of origin, can never achieve this level of informed protection, and generally requires users to solve a CAPTCHA on every post. Using Mollom's text analysis, users must only solve CAPTCHAs when Mollom is unsure about a post.  Read More >

Telling Humans and Computers Apart Automatically

A CAPTCHA is a program that can generate and grade tests that humans can pass but current computer programs cannot. For example, humans can read distorted text as the one shown below, but current computer programs can't:

Also you’ve probably seen them—colorful images with distorted text in them at the bottom of Web registration forms. CAPTCHAs are used by Yahoo, Hotmail, PayPal and many other popular Web sites to prevent automated registrations, and they work because no computer program can
currently read distorted text as well as humans can.

What you probably don’t know is that a CAPTCHA is something more than just an image with distorted text: it is a test, any test, that can be automatically generated, which most humans can pass, but that current computer
programs cannot pass. Notice the paradox: a CAPTCHA is a program that can generate and
grade tests that it itself cannot pass (much like some professors).

CAPTCHA stands for “Completely Automated Public Turing Test to Tell Computers and Humans
Apart,” and is used for the following.