Display “a generated one [avatar] based on their e-mail address”

The way gravatar works is that it creates an MD5 of the email address and passes that to gravatar.com. If an avatar exists for that hashed email address, then it gets returned.

If not, then it generates a random one based on the data in the hash. Essentially, it’s arbitrary. There’s code in there that basically says “if character 3 is A, use this color” and so forth. This means that the same email generates the same avatar every time, but still makes it randomish.

More info and examples: http://blog.gravatar.com/2008/04/22/identicons-monsterids-and-wavatars-oh-my/