Order Book

The order book is the live ledger of resting limit orders in a stock, buyers stacked at descending prices below the market and sellers at ascending prices above it. Every quote you see is just the top row of this ledger; Level 2 is the window that shows the rest.

Depth is the useful read. Where the size sits tells you how much buying or selling the market can absorb at each level, and large resting orders often function as intraday support or resistance. The caveat: the book shows displayed liquidity only. Hidden and iceberg orders do not appear, and posted size can be pulled in milliseconds, so the book is evidence, not a promise.