Akismet is a WordPressplugin which provides anti-spam functionality. We use Akismet in just about every website we build. In fact, Akismet is packaged up with every basic install of WordPress. The name Akismet derives from a combination of Automattic, the company that built Akismet, and the word kismet, meaning fate or destiny.
Akismet specifically targets spam arriving in the form of comments. It analyses each comment against a spam detecting algorithm and database. Any comment that makes it through this gauntlet is sent into a waiting area for you to approve or deny in the WordPress administrator interface.
You can install Akismet as a free version or a paid version with extra features. For most websites, the free version is perfectly adequate.
Another popular anti-spam plugin is WP-Spamshield. If your website is getting swamped with spam, you can utilise WP-Spamshield as an additional line of defence. It can run at the same time as Akismet.