Gravatars are globally recognised avatars. They are linked to an e-mail address, so every time you or one of your users posts on a Gravatar enabled site with that e-mail address their avatar is shown. If no Gravatar has been registered by the user, then a default is displayed (usually the grey ‘Mystery Man’).
WordPress 2.5+ has Gravatar support built-in, so no plug-ins required.
However, the Gravatar has to be created and linked to an e-mail address via the Gravatar website before it will show on a blog.
The Gravatar website is also where a Gravatar user can change their registered avatar picture for a different one.
See:
https://codex.www.ads-software.com/Using_Gravatars
and
https://en.gravatar.com/