Business process outsourcing (BPO) means contracting out specific business processes to a third party.
Business process outsourcing usually focuses on specific functions that are not part of the core business, but provide support to the core business.
An example might be the outsourcing of a call centre or customer-facing service. These external functions being outsourced is referred to as front office outsourcing.
When internal functions such as accountancy or payroll are outsourced, this is referred to as back office outsourcing.