Your grandma is either your mom's mom or your dad's mom. Some people call their grandma "Granny" or "Nana" or "Oma."
Grandma is a very common, informal way to say "grandmother." While the preferences of grandmas vary, many of them enjoy being known as Grandma to their grandchildren. Most people use the word grandmother when they're writing or speaking generally or formally. Grandmama came first, around 1749, and grandma followed at the end of that century.