Order Book (or market depth) — an electronic list of all open orders to buy and sell a specific financial instrument, such as stocks, currencies, futures, etc., on an exchange or trading platform.

The order book typically contains information about current bids and offers for the instrument, including:

Buyers (Bid): buy orders sorted from highest to lowest price. These are the prices at which market participants are willing to buy the instrument.

 Sellers (Ask): sell orders sorted from lowest to highest price. These are the prices at which market participants are willing to sell the instrument. 

Volume: the number of units of the instrument that market participants are willing to buy or sell at the listed prices. 

Price: the specified price at which participants are willing to execute a trade. 

The order book is an important tool for traders as it provides insight into the liquidity of the instrument and shows where buyers and sellers are positioned in the market. By using information from the order book, traders can make more informed decisions about entering and exiting trades.

For example, if there are many buy orders at a certain price in the order book, this may indicate strong support at that level. Conversely, a large number of sell orders at a certain price may indicate a level of resistance.