Humidity is a measure of water vapors in the air. Specific humidity is a measure of how much vapor is in a unit mass of air and can be calculated as the ratio of mass of water vapors to mass of air. In other words,
`SH = (m_w)/(m_a)`
where, SH is specific humidity, `m_w` is mass of water vapors in moist air and `m_a` is mass of dry air. Specific humidity finds applications in meterology. Rate of evaporation from a water body would be directly proportional to differences in specific humidity of water surface and air above it. Humidity can be measured using satellites (at large scale) or a psychrometer (at local level). One can buy devices, called dehumidifiers, to control the humidity of their immediate environment.
Hope this helps.