An order book is a record of all outstanding orders and their respective execution statuses within an exchange or trading venue. It contains information about the number of shares available at different prices, known as limit orders, as well as marketable limits that represent the highest price buyers are willing to pay (for offers) and lowest price sellers are willing to accept (bids). The order book is used by exchanges or trading venues to manage their books and match incoming orders with existing orders on the opposite side of the book. It helps in maintaining an efficient market, as it provides transparency into available liquidity at different prices levels, allowing participants to make informed decisions about where they want to place their orders.