Gravatar or global avatar is nice service that enables us to share our avatar anywhere. The main function of this service or application is to add our profile, including photos and short explanation about us into our web page. In wordpress this application is useful in comments. We can put our photos to our comments on our own site and also on the other site.
Gravatar is not enable by default in wordpress. It is on separate .com site. To enable this gravatar in wordpress u just need to signup to gravatar.com with your email address that u use in wordpress. there is the step to make it work.
1. Go to your wordress dashboard, Settings, choose General.
2. Check or fill the email address
3. Open new tab, www.gravatar.com
4. Sign up to gravatar.com with the same email address on your General Setting wordpress
5. Confirm your gravatar account on your inbox email
6. Log-in to gravatar.com, upload some images, Try make new comment, it will works
Some other explanation can u find in wordpress.org/codecs