Muggles are non-magical people. They are unable to use magic. They are also unable to see the Knight Bus, dementors, the Leaky Cauldron, and other magical places. They have objects which help them live a non-magical life. The Durseys are a very good example of Muggles. They are as mugglish as can be. Some muggles know the exsitence of magic, such as the parents of witches, the rulers of all nations and others.