An Order is an instruction sent to an Exchange or a Broker to execute a trade unconditionally, or when or if specific criteria are met

Detail

A Market Order is the simplest, unconditional, type of Order. It is a simple instruction to buy or sell a specific volume of a product or commodity to be executed immediately at the best price available

A Limit Order is an instruction to buy or sell a specific volume of a product or commodity if the price of execution is at or better than the Limit Price specified with the Order

An Order may combine, in a single instruction, a number of transactions that are required

A Fill or Kill Order requires all transactions to be carried out, or none. Partial execution is not permitted

Exchanges handle all types of Order internally

Other organizations or parts of organizations may accept combination Orders, and then route different parts of the Order to different Exchanges, Brokers or other parts of the organization

For example a large trading organization may have several desks issuing Orders that overlap. An internal order routing capability matches internal orders as far as possible before routing the unmatched orders to external organizations (brokers or exchanges)  

Bids and Offers are types of Order