26. Why is Queen Elizabeth sometimes referred to as Elizabeth Tudor? What does "house of Tudor" or "house of Stuart" mean?
Tudor was Queen Elizabeth's surname.
"House of Tudor" refers to the Tudor dynasty i.e all monarchs who were of the Tudor family. The Tudor dynasty began when Henry Tudor defeated Richard III at the Battle of Bosworth in 1485 and became King Henry VII. His son, Henry Tudor, became King Henry VIII in 1509, and his children, Mary, Elizabeth, and Edward, all had the surname Tudor.
A woman would only succeed to the throne if there was no son to succeed her, thus the succession of a woman meant that the dynasty was in trouble. During her reign, the dynasty would continue (as she would carry the name of her father, even if she married) but after her death, unless she had married a cousin of the same name, the dynasty would change to the surname of her husband (through her son or daughter) or to the surname of her successor.
This is what happened when Queen Elizabeth died. She was succeeded by James VI of Scotland, who bore the surname Stuart, and so the ruling dynasty changed from Tudor to Stuart. Therefore, we no longer had the House of Tudor, but the House of Stuart.