Klingon surnames indicate who an individual's father is, much like Russian patronymics. Worf's last name - if you choose to call it that - is son of Mogh.
Tuvok's last name is unpronouncable and unknown. At a publicity event way back in TOS days, a reporter asked Leonard Nimoy what Spock's last name was. He had no idea, so thinking fast, he said "You couldn't pronounce it." This bit of trivia became canon in "Journey to Babel," and there has never been a known Vulcan surname as a result. One of the ultimate in-jokes of Trekdom.