外公 refers to one's maternal grandfather.
外公 (wàigōng) cheat sheet for homework
姥爷 (lǎoyé): 姥爷 is another term for maternal grandfather.
爷爷 (yéyé): 爷爷 refers to one's paternal grandfather.
奶奶 (nǎinai): 奶奶 is the term for one's grandmother on either side of the family.