Diffraction is the phenomenon of how light bends around small objects or through small openings. These objects and openings have to be small enough to interfere with wavelengths of light, and so wavelengths in the red spectrum (longer wavelengths) are more affected by light in the bluer spectrum. Diffraction can cause a blurring of light, as well as causing interference in the transmission of invisible energies, such as radio waves and X-rays.
Most people think of spray cans when they talk about aerosols, but to a meteorologist any liquid or solid particle suspended in air is considered an aerosol.