Hercules Recaptcha uses the latest Google Recaptcha API to more accurately determine if users are bots or not. If the user is not logged in it will display a Recaptcha for the user to fill out in the comment form. If the user disables javascript and is not logged in then comments will fail to submit.
The Recaptcha is also added to the registration page for both multisite setups and single blogs. There are options for having it show up on comments and/or the registration page, as well as options for its position on the comment form, and which style (Google gives only two options, dark or light).