外婆 refers to one's maternal grandmother. She is the mother of one's mother.
姥姥 (lǎolao): 姥姥 is another term for maternal grandmother, just like 外婆.
奶奶 (nǎinai): 奶奶 refers to one's paternal grandmother, who is the mother of one's father.
爷爷 (yéye): 爷爷 means grandfather on either the maternal or paternal side.