What Is an Order Book? Bids, Asks, and Sell Walls Explained

What Is an Order Book?

An order book is the live list of every standing offer to buy or sell an asset on an exchange. Bids on one side, asks on the other, sorted by price. The market price is nothing more than the point where the two sides currently meet. Everything else people say about price is commentary on this list.

How It Actually Works

Bids are standing buy orders below the current price. Asks are standing sell orders above it.
A market order consumes the book at the best available prices. A limit order joins the book and waits.
Depth is how much size sits near the current price. Deep books absorb big trades calmly. Thin books gap. This is liquidity made visible.

Buy Walls and Sell Walls

A wall is an unusually large cluster of orders at one level. A sell wall sits above price and absorbs rallies until it is either eaten or pulled. A buy wall sits below and cushions dips the same way. News coverage loves describing overhead supply, like long-dormant coins returning to exchanges, as a wall choking a market: a mass of standing sell interest that every rally has to chew through before price can rise.

The honest caveat: walls can be theater. Large players sometimes post walls to spook the market and cancel them before they fill, a manipulation called spoofing. A wall is real only when it gets eaten and holds. Watch what fills, not what displays.

Risks and Common Mistakes

Trading around displayed walls as if they were commitments. They can vanish in one click.
Market-ordering size into a thin book and paying for the whole staircase. That cost is slippage.
Forgetting that DEXs work differently: pools reprice by formula rather than by matching a book. See DEX.

When It Matters

Reading depth before placing any large order, and decoding market commentary about supply overhead. A current example, Bitcoin grinding under a wall of returning supply, is worked through on my learn page about sell walls and thin volume.