Aerosol Direct Effect
Aerosol affect the Earth's climate directly, by absorbing the sun's light or reflecting it back into space. Dark aerosol like black carbon or dust will absorb light and have a warming effect, while light-coloured aerosol like sea spray or sulphate reflect light and cool the planet.
Aerosol Indirect Effect
Large aerosols (> 100 nm) can act as seeds on which cloud droplets form. Liquid water clouds have a strong cooling effect on the Earth's climate. When more aerosol are present in a cloud with a given amount of water, more cloud droplets will form. These droplets will be smaller. The result is an increase in the cloud's albedo; polluted clouds reflect more light into space, and cool the climate more effectively.
The cloud will also have a longer lifetime, because smaller droplets are less likely to rain out. The aerosol indirect effect is larger than the direct effect, and more difficult to quantify.