A centralized exchange order book lists limit orders to buy and sell an asset at different prices. The best bid and ask form the quoted spread, while the depth at each price level affects how a larger order may execute.
The spread is the gap between best quotes
The highest available bid is what buyers are currently offering; the lowest ask is what sellers are asking. A market order can cross this spread and fill against multiple prices.
Market orders prioritize execution
The final fill can cross several order-book levels and differ from the last-traded price.
Displayed depth can change before execution
Orders may be cancelled or updated, and liquidity can differ across venues and pairs. Market orders prioritize execution rather than a fixed price, so slippage is possible; limit orders control price but may not fill.
Limit prices constrain execution, not certainty
A limit order may remain unfilled if the market does not reach its price.
Sources
Educational information, not investment advice.