An avatar is a graphical representation of you. It doesn’t have to be a photograph, it can – for instance – be a company logo. If you haven’t uploaded your own, and don’t have a Gravatar associated with the email address you used to register, you’ll be represented by the default avatar shown on the left. You can change this in your dashboard.
A Gravatar is a particular brand of avatar associated with an email address that then gets used on any sites that use Gravatar when you use that email address. You can learn more about Gravatars, and set up your own if you wish, at gravatar.com. A Gravatar associated with the email address you used to sign up with will take preference over the default one, but will in turn be overiden if you upload a specific one for this site.