The document discusses drilling fluids, defining them as circulating fluids used in rotary drilling operations and outlining their historical development from ancient times to present-day advancements. It categorizes drilling fluids into various types, such as water-based, oil-based, synthetic-based, all-oil, and pneumatic-drilling fluids, each with distinct properties and applications. Additionally, the document highlights the functions of drilling fluids, their advantages, and disadvantages associated with their use in drilling operations.