If you're like me and of mixed ethnic origin, the answer is , "a lot of things."
Here in Canada we called my Irish grandfather Grampa -- though the Irish tradition would have been Granda.
My Italian grandparents were called Ma and Babbo (Italian for daddy) because of a generation overlap. You see, my eldest cousins were living in the same house as my grandparents and my youngest Aunt. Naturally she called her parents Ma and Babbo. And so did my cousins. So... we all did. Ma was Ma to her great-great-grandchildren. We called Ma and Babbo's mothers Nona. I know proper Italian is "nonna." Was Nona a spelling from Furlan or the Marciagian dialect? Or were we just uneducated?
Whatever the reason, Nona lives on. When my daughter, "the Bean," was born, we decided that she would call my Sweetie's father Grampa, since her elder cousins had set that in stone. And my parents would be Nona and Granda. The Granda thing didn't stick. It's hard for the North American tongue. Granda became Grandad. For a while. Then the Bean started talking. Her vocabulary started to flesh out at around 20 months. That Christmas, my parents were Goo and Gung. At that point, Gung meant Granda, Grampa, guy with gray hair, and a pink Beany Baby bear (who knows?). Eventually though, my mom became Nona. And Gung got narrowed down to my dad.
My brother's since had his own sweet little girl, "Little Woman". He tells her that our parents are called Nona and Grampa. We'll see. The Bean says he's Gung. And I can already tell Little Woman has a mind of her own.
What do you call your grandparents? And why?