As a noun grandparent
is the parent of someone's parent.
As a proper noun grandma is
the name used for one's grandma.
Other Comparisons: What's the difference?
the parent of someone's parent
* (by grandparent's gender ) grandmother, grandfather
* (by parent's gender ) maternal grandmother, maternal grandfather, paternal grandmother, paternal grandfather
* (with regard to ancestry ) grandchild
* great grandparent
* great great grandparent