Captcha! plugin Manual
Many spammers use bots to automatically flood your blog with lots of comments with commercial advertising (e.g. gambling sites, or porn sites). The worst of all is that neither you nor your readers want to know about these sites. They just want to comment about your post and read other commenters opinions.
But since spam bots populate your blog with lots of comments, users will find true comments hard to read among so much spam, and they might end up discouraged and even stop visiting your site. And since spam comes in very often (due to automatic Internet Robot programs), you will soon find spending too much time deleting them.
There are several ways for trying to stop Spam in your WordPress blog, and yes, Captcha! is one of them.
What's a Captcha?
Captcha are test to guess if the user is a human or a program (See Wikipedia related article).
This plugin implements a visual captcha, but there also are audio captchas (a sound you have to understand to give an answer) and other test (like simple questions). Each of them has Pros and Cons.