A work permit system is a formal written system to control certain types of work identified as potentially hazardous. The terms "P.T.W.", "permit" or "work permit" refer to the form used by a company to meet its needs. These systems aim to ensure proper planning and consideration of the risks involved in a particular job, at a specific time and place, with designated precautions.
It is usually categorized in "hot" work permits and "cold" work permits. Hot works are those where there is a potential of generating fire or extreme heat, cold ones are all the others. (wikipedia