Onigiri is often called a rice ball in America. I'm not sure of the technical definition but they can also be called Musubi or Omusubi. If I understand correctly, musubi means "tied up" so I would think that maybe originally it referred to the kind of rice ball with a little strip of nori running around the center but as you can see below, some of the ones labeled as musubi have no piece of nori.
My understanding is that basically Onigiri is rice that's been pressed into a ball after putting some water on your hands and a little salt so that as you press it into a ball you add some favor do it. Of course that's the base. You can add anything you want to it from using different spices in the rice to adding stuffing and or wrapping it with a piece of nori or even scrambled egg or other wrappings.