Order book

An order book is the live list of buy orders (bids) and sell orders (asks) waiting on an exchange for a trading pair, such as BTC/INR, organised by price. The highest bid and the lowest ask form the top of the book, and the gap between them is the spread.

When you place a market order, it fills against the best available orders on the other side. If your order is larger than the amount offered at the best price, it moves on to the next levels, which is how large orders cause slippage. Limit orders that do not fill immediately are added to the book and wait for a counterparty.

Looking at order-book depth tells you how much can be traded near the current price. Note that some displayed orders may be cancelled before you reach them.