Gravatar is the abbreviation for ‘Globally Recognized Avatar’. Everyone can set their custom avatars through the Gravatar website – http://gravatar.com/. Once you set your avatar, it will be displayed alongside your comments in any website that supports Gravatar.
In order to enable Gravatar in your WordPress comments, you must install the ‘WP-Gravatar’ plugin. After activating this plugin, the Gravatars will be displayed next to the username of the people who comment on your posts. Moreover, you can also set a customized image that can be displayed if the user does not have a Gravatar account.
By enabling Gravatars on your website, the users can also post comments without being registered. Along with this, an advantage for you is that you will be able to recognize the regular visitors to your website through their Gravatars.
Note that Gravatars match the email ids used by the users for registration.