Is the top of your dresser crowded with knickknacks? Then it's cluttered. If you clean up your cluttered desk, you might find that history paper you were supposed to turn in a week ago.
Sometimes cluttered things are dirty, disorganized, and filled with rubbish, like your neighbor's cluttered garage, stuffed with broken tools and garbage. Things can also be cluttered in a clean but overstuffed way, like your grandmother's china cabinet, cluttered with little cat statues. This adjective comes from the verb clutter, originally "to collect in heaps," and later "to litter or crowd."