A company that registers icons against your email address. When you visit any site that supports gravatars, such as richarddawkins.net your currently registered icon is made available to them. Gravatars can be photos, art, even X-rated. You can also register you home website, contact info, additional pictures, a biography… things you want the public to see if they click your icon. Make sure you click save after entering data or it will all be discarded. I suspect even if you don’t give your email address to a site, it can still access the Gravatar information. Sites I have never visited before seem to know my logo even before I tell them my email address. Perhaps the get it via my IP.
Icons used to intentify posters on forums are called avatars. Avatars can be registered centrally so you don’t have to set them up on every forum. When you do that, they are called gravatars.
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