Fetching Gravatar

How can I fetch anyone’s gravatar image?

I need to show users gravatar image on a custom profile page that am making.

I have so far integrated the option of Facebook Profile Pic (which automatically gets grabbed) and also am allowing user to assign his/her own photo (on wish).

What I need to do is, if person do not comes from FB, then maybe his email or something should be used to see if he/she has a gravatar, if yes then display it. How can I do that?

Thanks.