A simple definition of this would be "the opposite of my bedroom." But technically speaking, a clean room is a highly purified and regulated room for producing equipment sensitive to contamination. For example, the production of microprocessors is done in a clean room, since the smallest amounts of dust could create flaws in their production. Therefore, the air in a clean room is constantly filtered. Other aspects such as temperature, humidity, and pressure may be regulated as well.
Clean rooms are rated by class, according to a Federal standard. A Class 1,000 clean room has no more than 1,000 particles larger than 0.5 microns in a cubic foot of air. A Class 100 clean room has no more than 100 particles. The people who work in clean rooms need to wear special suits that prevent them from contaminating the room. They look almost like astronaut suits, but are not quite as heavy and don't require the large boots for walking on the moon.