Order Book
An order book is the live list of all open buy (bid) and sell (ask) orders for an asset, sorted by price.
In depth
The highest bid and lowest ask define the spread; a trade happens when someone accepts the other side’s price. Depth — how much volume sits at each level — shows how much can be traded without moving the price. Reading the book tells you where liquidity really is.
Why it matters
The order book is the engine of price discovery on any spot or futures market.