A Gravatar is a Globally Recognized Avatar, used as an identifying image on blog posts, comments and similar. The concept is a simple but clever solution to the problem of how you get an image linked to someone's email address without exposing that email address to the public. A naive solution could be the following:
The trouble is, the email address is there in the HTML ripe for indexing and spamming.
Gravatar uses the simple approach of hashing the email address, so it is not exposed:
This gives the result below for one of my email addresses. Note that the address isn't exposed, I can happily post it here without inviting a deluge of spam. I've also included the gravatar of a commenter, again not exposing their email, and shown the neat "unknown" image returned if the gravatar doesn't exist.
The other point of course is that the Gravatar site is a widely known solution for blogs, and part of WordPress, so the most likely avatar source is easy to pinpoint.
Out of curiosity I wrote a quick online tool to show gravatars for a big set of emails, and put in the contents of my address book. Of two hundred people only a handful have gravatars, which compares very poorly with the number who have a Facebook account, suggesting Gravatar is still a fairly niche solution.