Akismet is the build in plugins that provide the extra functionality to help you to fight with the comment spam in your wordpress blog. Akismet will checks the comments that you received from the reader with the Akismet web service to see is the comments look like spam or not.
To enable Akismet, go to install plugins, click on activate Akismet. You need the wordpress API key (More on wordpress API key). You can get the API from the wordpress dashboard.
Akismet is brilliant because you just activate it in the beginning when you start your blog and it just sits there and does its thing.
Click here to go to Akismet home page