A muggle is what wizards call non-magic folk. They are people, who have NO magic in their blood at all. Most witches and wizards do not approve of muggles and it is the same vice-versa. If you are having a hard time figuring out who is a muggle, think of the Dursley's or Hermione's parents. In the wizarding world, the wizards have many anti-muggle protections against wizarding events, objects, and places. One of the precautions is if a muggle sees any magic, instantly, a memory charm will be put on him/her so that he/she will forget what he/she saw.