The order book is a list of all open buy and sell orders that are waiting for execution. Orders to buy are called "bids" and orders to sell are called "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. Depth chart is a constantly changing display showing the total number of orders to buy and sell an cryptocurrencies.