Order book
An exchange's live list of standing offers to buy and sell at stated prices, which is where traded prices actually come from.
An order book is the running list every exchange keeps of who wants to buy and sell, at what price, and in what quantity. Bids stack on one side, asks on the other, and the traded price is wherever the two sides meet.
The number a ticker shows is only the top of the book, the best current offer. What the ticker does not show is depth: how much is actually available at that price. Ask for more than the top level holds and your order walks down the book, filling at worse prices level by level, which is where slippage comes from.
Depth varies enormously. Bitcoin's books refill around the clock across hundreds of venues. A small token might have one shallow market where a single £5,000 order visibly moves the price. The same headline can barely dent one coin and send another up 40 per cent by evening, and book depth is a large part of why.
Resting orders queue by price first, then by arrival time. Your first crypto purchase through an on-ramp never touches a book at all: that model quotes you one price instead of auctioning your order.