Dust is formed from the finest, smallest particles of dirt and sand that become airborne due to the forces of erosion present on the Earth. Dust is a natural phenomenon that, unfortunately, has to be dealt with in virtually all habitable homes and buildings. Normal household dust can be composed of a multiplicity of things, such as the aforementioned small dirt particles. People are losing skin cells all the time, so the shed skin cells contribute to the composition of the dust. Dust mites are small, arachnid-like creatures that feed off dead skin cells. Both they and the waste products of their existence contribute to the dust aggregation. Pets in the household shed fur, which becomes the rolling "dust-bunnies" that are so hard to chase down and collect. Those same pets bring in fleas, which are small, parasitic creatures that feed off the pets, living and dying amid all the dust accumulation. So add all these sources together, and you get a nice, finely-laid layer of dust that has to be cleaned on a periodic basis.