Order book
An order book is the live list of every resting buy order (bid) and sell order (ask) for one instrument on one exchange, sorted by price.
The order book is where a market’s buyers and sellers wait. Bids sit below the price, highest first; asks sit above it, lowest first. A trade happens when an incoming order accepts a resting one, and the exchange matches orders by price first and time second.
Each exchange keeps its own order book, which is why the same instrument can trade at slightly different prices on different venues. Where large orders rest, price tends to stall; where few rest, it moves quickly.
Learn more: the order book: how a price is made and practice: read your first order book.