Akismet is a spam-fighting service that is different from others such as Spam Karma 2 or Bad Behavior in that it checks the content of the comment anonymously with an online server, to determine whether it is spam or not.
The Akismet WordPress plugin ships with WordPress 2.0 and later by default, but if you don’t have it for some reason, you can download it from the official page.
After downloading, go to the Plugins page in your administration panel, and click the Activate link in the same row as the Akismet plugin entry. After you’ve activated it, it will come up with a box below the main navigation saying that the Akismet plugin is not active, and you need to enter your WordPress.com API key.