An Order Book is an electronic list of buy and sell orders for a specific asset with an exchange or a market that are still open. These are effectively organized by price.
The Matching Engine, a software responsible for matching buy with sell orders, makes use of the Order Book to launch trades for participants of an exchange. Considered the core of all the exchanges, the order matching system is responsible for determining the effectiveness of the exchange. Across, Order Books usually keep the same information, the only noticeable differences are seen through the layout, depending on the platform used by the traders.
Order Books are considered useful by traders in that they assist in measuring the interest of buyers and sellers in light of price levels. The data provided by Order Books can bear valuable information regarding the possible support and resistance levels.