Akismet is considered one of the essential plugins. It comes with every WordPress installation so you only need to activate it. The complication here is that it requires the API key from WordPress.com to work so simply sign up there and get the key. When you give it the key the plugin will start working.

What Akismet does it catch spam comments and hold them in a separate queue. This makes the process of removing comment spam much simpler. If a comment someone avoids Akismet you can classify it as spam and Akismet will learn from this and not let it through the next time.

It’s completely worth the hassle of getting the API key. Install it immediately.

http://akismet.com/