An order book is a digital list that shows all the buy and sell orders for a specific asset, such as stocks or cryptocurrencies. It helps traders see the current market demand and supply, displaying prices and quantities that buyers and sellers are willing to transact at. The order book is crucial for market participants, as it provides transparency and helps in making informed trading decisions. It is often used in conjunction with trading platforms and exchanges, where users can place their orders based on the information available in the order book.