An order book is the live list of resting buy and sell limit orders on a venue, usually sorted by price. Bids sit below the mid. Asks sit above. The gap is the spread. Stacked size away from the mid is depth. Traders read the book to judge how much they can buy or sell before moving the price.
Why it matters
Charts show where trades already happened. The order book shows where resting liquidity claims to wait. Thin books turn small market orders into ugly slippage. Thick walls can look like support or resistance until they cancel. Spoofy size and fleeting quotes mean desks trust fills and trade tape more than a single screenshot of depth.
Crypto books fragment across exchanges and pairs. The deepest BTC/USDT book on one venue may not match a shallow USD book elsewhere. Funding and futures books can diverge from spot. When you size a move, name which book you are using and whether withdrawals or API limits could strand you after the fill.
Market makers and high-frequency flow refresh the book constantly. A wall that vanishes as price approaches is not a promise. Pair book reading with liquidity and market depth vocabulary, and remember that on-chain pools are a different microstructure even when they compete for the same ticker.
Order-book readers also watch refresh speed and pull rates. Size that appears and vanishes with every tick is advertising, not inventory. Persistent resting liquidity that absorbs flow without canceling is closer to real depth. That distinction matters more than the tallest wall on a screenshot.
Example
An alt shows a large sell wall a few percent above the last trade. Social chat calls it a ceiling. You watch whether that size stays as price rises or pulls before being hit. If the wall cancels and the book thins, the “resistance” was never real resting supply. The next market buy walks further than the screenshot suggested.