Image for your online comments
Gravatar is a small image that is associated with your email, and every time you write a comment online where the gravatar service is supported the site gets to pickup your picture and use it as your signature.
Gravatar is a good way to get a face to the person behind a comment, but it could also be a logo, if you are commenting on behalf of a company.
Another neat thing is that every unique gravatar is associated with an email, so if you have multiple emails, you can have multiple online avatars :-)
See my comment below if you create a gravatar and write a comment, your picture also appears.