When someone makes a comment in your posts, you can see only his or her name. Any person posting comments using your email address must be you. So in whichever blog you make comments using an email address that belongs to you, it indicates that the person who made those comments is you. So an email address is a unique ID of a person.
When someone makes a comment in WordPress, by default you can only see his or her name and email address. It will be more pleasing if you can see his or her avatar in this comment. An avatar is a small image chosen by someone as his or her picture. It may be an image of anything. However, usually a person uses as an avatar a picture that he or she thinks best represents himself or herself.
Gravatars came from this idea. Gravatar means “Globally Recognized Avatar”.
Gravatar.com is mainly a site that hosts your avatar as a URL from its site. You can upload your avatars into the gravatar website and get an ID against your email account. You can change your avatar anytime. You can also rate your gravatar for different audiences like “G” or “R” as suitable to all.
When someone who has an account in gravatar against his/her email address makes a comment in a blog, the blog engine searches the gravatar repository with his or her email address and finds the avatar. Then that blog engine displays the avatar (or gravatar) in every comment he or she has made in the blog.
This is a fantastic feature for every blog engine. Gravatars are known worldwide and almost all the big blog engines have the capability to include this feature. From the gravatar site, there are plug-ins available to add this feature to different blog engines. There is also a plug-in for adding this feature to WordPress.
It also adds to the look of a blog. Gravatar was started by Automattic, the company that created WordPress.