If you speak a simple way, we can answer the cleanroom in an enclosed room in which the volume of dust in the air which is limited in its minimum level aims to avoid causing the process to the soil for research, production and many projects/industrial turned.
At the same time, temperature, pressure, and humidity of the airs are also mineral and control mechanisms to have the most benefits for the above process. Also, be sure sterilized, do not have the proper according to toxic air means "clean" of it.
For cleanroom usually, solve the main problem is the temperature - pressure room - cleanliness problem of cross-contamination.
The cleanroom standards were first launched in the US, now has become the world standard. It is standard on dust content in a unit volume of air, the class is divided particulate size and type of rooms is determined by the size of dust particles 0.5μm brightly on per unit of volume 1 cubic feet (ft3) of air in the room.