You are encouraged to visit the Akismet site to learn more about it. In just a few words, when new content is submitted to your site, the akismet module sends a real time request to Akismet where hundreds of tests are applied to get a thumbs up or thumbs down kind of response. It may finally that tell us if the content is spam or not. If it isn't, the content is published, if it is, then it is placed on the moderation queue. Moderators may opt-in to receive e-mail notifications for all content, just content needing approval or nothing at all. The module is fully customizable, almost everything is optional. It also keeps track of a counter that you can use to show your visitors how many spam has been caught by Akismet in your site.
I have also implemented a quite simple method to check for updates at certain intervals, which is customizable from the settings panel. It can also be disabled.
For a complete list of features, support resources and download, please visit the project page for the Akismet module for Drupal.
Please note that in order to use the Akismet Service, you need a WordPress.com API key. If you don't have one already, you can get it by simply signing up for a free account at WordPress.com. Please, consult the Akismet FAQ for further information.