Akismet is used for detecting and filtering out spam in comments, trackbacks, pingbacks and contact form messages. The plugin was developed by WordPress co-creator Matt Mullenweg in 2005, when comment spam was a big problem in the WordPress ecosystem. It is owned and maintained by the company Automattic, which Matt Mullenweg founded a couple of months after Akismet launched. To date, Akismet has blocked more than 430 billion spam comments. It is one of only two plugins that come bundled with every WordPress installation. The other one is Hello Dolly.
The way Akismet works is that it maintains a centralised database of spam comments. Whenever a comment is posted on a site with Akismet activated, the comment is checked against the central database to determine whether it is spam or not. If it doesn’t get flagged by Akismet, but is marked as spam manually by the site owner, it gets added to the database. The next time that comment is posted on a site with Akismet, it is flagged as spam automatically. That way, the spam filter gets more effective over time.
Akismet is available in both free and commercial tiers. The free tier can be used by personal sites and blogs, while the commercial ones offer priority support and advanced stats and are meant for commercial websites.