The back office is the portion of a company made up of administration and support personnel who are not client-facing. Back-office functions include settlements, clearances, record maintenance, regulatory compliance, accounting, and IT services. For example, a financial services firm is segmented into three parts: the front of(e.g., sales, marketing, and customer support), the middle office (risk management), and the back office (administrative and support services).
A back office in most corporations is where work that supports front office work is done. The front office is the "face" of the company and is all the resources of the company that are used to make sales and interact with customers and clients. The back office is all the resources of the company that are devoted to actually producing a product or service and all the other labor that isn't seen by customers, such as administration or logistics. Broadly speaking, back office work includes roles that affect the costs side of business' trading statement and front office work includes roles that affect the income side of business' trading statement.