An order book is the list of every open buy (bid) and sell (ask) order for an asset, displayed by price level. The highest bid and lowest ask define the current market spread; the difference between them is the bid-ask spread. Order books are the core data structure of most centralized exchanges (stocks, crypto, futures) and stand in contrast to AMM models that use formula-based pricing.
When a trader places a limit order, it is added to the order book at their specified price. When a trader places a market order, the exchange's matching engine walks the order book from the best price outward, matching against resting orders until the market order is filled. The matching engine sequences thousands to millions of events per second on high-volume exchanges.
Order books reveal market depth (how much volume sits at each price level), which signals liquidity and likely slippage for large orders. "Iceberg" orders hide most of their size and reveal only a small portion at a time, to avoid moving the market.
Order book depth is one of the strongest signals of market health. Thin books spike with volatility on small trades; deep books absorb large trades with minimal price impact. Reading the order book is foundational skill for active traders.
The whale-tracker premium endpoint and the macro endpoint provide context that complements order-book reading on individual venues.