Order Book in one sentence: An order book is a live list of buy and sell orders showing where traders are willing to bid or offer.
An order book is the matching layer behind many crypto trading screens. On Hyperliquid, perp markets use an order book, so you will see bids below the current price and asks above it. Beginners usually notice it as a constantly changing ladder of red and green numbers next to the chart.
How it works
Buy orders, called bids, state the price and size buyers are willing to pay; sell orders, called asks or offers, state the price and size sellers are willing to accept. When a market order arrives, it matches against the best available orders on the other side until the requested size is filled. The gap between the best bid and best ask is the spread, and deeper books usually handle larger trades with less slippage.
Why it matters
The order book helps you understand why a trade fills at one price instead of another. If liquidity is thin, a large market order can eat through several price levels and create worse execution than the chart suggested. Beginners can use limit orders, smaller trade sizes, or TWAP-style execution to reduce unnecessary slippage when the book is not deep.
Use it in a sentence
Example: “The order book looked thin above $3,000, so I used a limit order instead of market buying the full ETH position.”