BPO means Business Process Outsourcing. It is the process of hiring a company which is capable of doing jobs on your behalf at better expertise and much lesser price so that you can focus on your principal job.
When a subsidiary of parent company runs operations of the outsourced work subsequently the BPO is called prisoner. For example: Dell decided to start its own call centers in India that it can manage or make calls for Dell products.
When the outsourced work has been handled by a third party which has been paid for its services then that third party company is a non prisoner unit. For example: Wipro manages projects of various ISPs,banks, computer manufacturers etc.