An Aerosol is a solution of a liquid in gas.  Aerosols consist of minute particles of liquids which are floating in gas.  Clouds, fog and mist are examples of natural aerosols. 

Other examples of aerosols, we come across in daily life, spray painting, perfume sprays, pesticide spray guns, etc. 

Aerosols find wide applications in the field of medicine.  Many medicines, such as those for asthma, are delivered as aerosols.  Aerosols are effective in delivered precise amounts of medications to the correct location using metered dose inhalers. 

Compressed air is used as the propellant in many applications.  In medicine, aersols are delivered using hydrofluoroalkanes.