An orb is a ball-shaped object. You might be inspired to write a poem about the full moon in which you describe it as "a glowing orb."
The sun, moon, marbles, grapes — all of these have been described (particularly by poets) as orbs. Your round gold earrings, your dog's favorite tennis ball, and a grapefruit before you cut it in half can be called orbs too, because they're all spherical in shape. The Latin source, orbem, means "circle, disk, ring, or hoop," and is probably related to orbita, "wheel track or rut."