The Steem blockchain offers an internal market that works as decentralized exchange. The internal market allows users to trade their STEEM, SBD and SMTs through a public decentralized peer-to-peer market. Users are able to place buy and sell orders, and order matching is performed automatically by the blockchain. There is also a publicly accessible order book and order history which users can use to analyze the market. Users can interact with the exchange directly using the blockchain API, or use a GUI such as the one on Steemit.com
A buy order is a request to buy on the internal market.
A sell order is a request to sell on the internal market.
An order book is a list of buy and sell orders organized by price level. An order book lists the number of tokens being bid or offered at each price point or market depth. The blockchain also identifies the market participants behind the buy and sell orders, although the UI may choose to not display the information (the information is publicly available on steemd)
An order history shows all market participant open orders and canceled orders. If an order price is not met, it remains in the order history until those conditions are met or canceled. Not to be confused with trading history that only shows buy and sell orders that were successfully matched.