An "Order Book" is a list that shows all the buy and sell orders for a specific asset, such as stocks or cryptocurrencies. It helps traders see the current demand and supply in the market. Each order includes details like the price and quantity of the asset that traders want to buy or sell. The order book is essential 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 displayed in the order book.