Clean Rooms are specified by their class of cleanliness. While there are many factors, the class system allows people to use a generally similar frame of reference. In the UUS most people use Federal Standard 209E which defines the classes, how to measure them and how to report them. Until you get to a real clean facility, the reference particle size is 0.5 microns. By measuring how manysuch dirt particles are in a cubic foot of atmosphere, you define the class. The bigger the number, the dirtier the room.

For instance:
CLASS 100,000 (100,000 0.5 micron particles per cubic foot of atmosphere)
Typical for a computer room. Besides restricted access and positive pressurization, no special garments or procedures are required.

CLASS 10,000 (10,000 0.5 micron particles per cubic foot of atmosphere)
Typical for a hospital operating room. Some types of factories use this class of room. Clean Room coats or bunny suits, head covers and booties are required. Air enters the room through HEPA filters. (High Efficiency Particle Accumulation) Access is restricted. Room is positively pressurized relative to surrounding space.

CLASS 1,000 (1,000 0.5 micron particles....you get the idea)
Typical for making computer disk drives or other sensitive materials. Same rules as Class 10,000 but workers are required to wear gloves and full hoods as part of the clean room wardrobe. More frequent air changes in the room. Again, all air is constantly filtered through HEPA filters and the number of air changes per hour is very large.

CLASS 100
Typical for special semiconductors like computer memory. Also used for some pharmaceuticals. Same idea as above, just more stringent. Full face masks are required here.

CLASS 10 to CLASS 1 and cleaner
Good for large semiconductor fabrication where even the slightest amount of contamination will cost a lot. Workers wear complete suits, with face masks. Breathing is done through a respirator to filter workers' breath and prevent breathing from contaminating the space.

In general, all of these facilities have the air directed from HEPA filters straight at the work. The term is "product gets first air". This means that the air is the cleanest as it leaves the filter and that cleanest air should come in contact with the product being manufactured before coming in contact with anything else especially people. The closer the product is to the source of filtered air, the more likely it is that the product will be defect free.