The reCaptcha is a Google's service which purpose is preventing spam on blogs, forums and portals.
There is also reCaptcha WordPress plugin that can be easily installed and integrated into your website in order to block spammers. It also uses MailHide to stop email spam.
How to install reCaptcha WordPress plugin
1) Download the compressed reCaptcha WordPress plugin
2) Extract the folder into wp-content/plugins folder of your WordPress installation
3) Go to Admin > Options > Plugins Management and activate the plugin
4) You will be prompted for a public and private API key. You can sign-up for the keys at the reCAPTCHA site using the link provided, and then enter them in the text fields to activate the plugin.
5) reCaptcha widget will appear on the comments page
If you still seeing comment spam don't worry, they are mainly moderation emails, trackbacks and pingbacks.
In case that you need to update reCaptcha keys, navigate to WordPress Plugin Management page where you can see a tab for reCaptcha options including API key.
Sometimes, the plugin is missing from the plugins management page. It could be caused with wrong permissions so change the reCaptcha directory permission to 755. Do not change the contents' permission that should be 644.