Junk is trash, debris, or garbage — it's something that's left behind or thrown away because it's not wanted. You might view the broken toaster as junk, while your sister sees it as an interesting repair project.
After an accident, your car might be junk, and if it's hauled off to a scrapyard, you can say you had to junk it, using the verb meaning "to throw it away." One person's junk, however, is another person's valuable metal that can be sold to recycling companies. A completely different, non-trashy kind of junk is a Chinese sailing ship. These uses are connected through the nautical origin of junk, originally spelled junke and meaning "old rope or cable."