Hi...
A call center is an example or a type of a BPO (Business Process Outsourcing) unit.
Call centers are those where telephone calls are being handled that may be inbound (called by customer for getting information about a product) or outbound (employees of center call people to sell some product or to provide information about some service) whereas BPOs are those where any type of outsourcing work is done. A BPO may be doing HR outsourcing job for various companies, handling accounts for different companies etc.
Hope that you have got some help in differentiating BPO and a call center.
VISHAL