As the title kind of suggests, Gravatar has to do something with avatars, or your own image.
There are plenty of blogs out there in the web, and in almost all you can leave comments. Some blogs use Facebook accounts, others use various open source systems, and some just use their own custom system with gravatars, like this site does.
You can read all the details about gravatars on their page http://www.gravatar.com
But in short, it is a simple system, where you upload your photo and "connect" it with your email. Now whenever you enter your email when posting comments in blogs that support gravatars, your gravatar will appear next to your comment.
If you check below, you will see the "Christmas addition" I was talking about. Yes i finally added the comments, so you can write your own thoughts about the content of the articles.
As of yet, comments still aren't spam-safe, so spammers and bots please behave, until I find more time to update the system.
And do not worry, your emails are safe, they are used only for displaying avatars.
For now happy commenting and marry Christmas :)