It works like this: whenever a comment, trackback, or pingback is posted on your site, it's sent to the Akismet website, which tests it and sends back either a favorable or unfavorable result.
With Akismet, you don't need to keep a black list of spammers any longer. This web service takes care of everything.
If you happen to receive a false positive, you can always get in touch with Akismet to get a different result.
To use Akismet, you need an API key, which you can get on the user registration page on WordPress. Click here to access it.