The order book is a list of all open buy and sell orders that
are currently pending execution. Orders to buy are known as bids and orders to sell are know as asks. Bids with the highest price will be at the top of the bid list, since they will be the first ones to be executed. Conversely, asks with the lowest price will be at the top of the ask list for the same reason.
The graph on this page is know as a depth chart. This chart graphs the amount of Bitcoin that is available to be bought or sold at a specific price.