Materials start to behave differently at the nanoscale. The bulk materials that we have traditionally dealt with, like metals and plastics, are uncontrolled and disordered at small scales. The strongest alloys are still made of crystals the size and shape of which we control only crudely. By comparison, a tiny, hollow tube of carbon atoms, called a carbon nanotube, can be perfectly formed, is remarkably strong, and has some interesting and useful electrical and thermal properties.
When particles get small enough to qualify as nanoparticles, their mechanical properties change. The way light and other electromagnetic radiation is affected by them changes. Using nanoparticles in composite materials can enhance their strength, reduce weight, increase chemical and heat resistance, and change the interaction with light and other radiation. While such composites have been made for decades, the ability to make nanoparticles out of a wider variety of materials is opening up a world of new, stronger and cheaper composites.