If everyone decided to litter, or drop trash on the ground, the world would be covered in garbage. Litter doesn't always have to be trashy though. When a cat has kittens, they are born as a litter.
Don't be confused when you go to England and see the word litter written all over garbage cans. That's just the Brits' way of saying trash. The word litter comes from the Old French litiere meaning "bed." Animals like dogs or cats have their offspring all in one birth, or on the same bed, making what we call a litter, or group of babies.