Aerosols are small particles that are lofted into the atmosphere by the effects of trees, volcanos, and human enterprises like farming, driving, and building. Aerosols in the air can affect the climate in a number of ways. If they are hydroscopic they can attract water vapor and become larger, affecting visibility and how much terrestrial radiation is absorbed as it exits the earth. If they are dark they can reflect sunlight back to space before it gets to the ground, cooling things off, but if they fall on white snow they can make the snow darker and help it to melt faster, changing the local albedo. That also changes the radiative balance of the earth, which changes its temperature. Understanding how aerosols affect climate in different ways is one of the keys to having a good understanding of how climate might change in the future. Science magazine has a good new article which explains this in fairly readable form at http://science.sciencemag.org/content/360/6385/148.full.