A microsphere is any sphere, solid or hollow, that has a diameter that is on the length scale of micrometers.
Microspheres have found various applications in the field of biology, medicine, electronics, and ceramics. For example microspheres have been used to target sites for drug delivery. Also they can be used to decrease the density of a material by being injected into that material. In fact, most people have probably used polystyrene microspheres as exfoliators in hand creams and lotions.