Hi,
A call center is an example of a type of a BPO (Business Process Outsourcing) unit. Call centers are those where telephone calls are being handled, which may be inbound (called by customers for getting information about a product) or outbound (employees of the center call people to sell a product or provide information about a service). On the other hand, BPOs are where any type of outsourcing work is done. A BPO may be doing HR outsourcing jobs for various companies, handling accounts for different companies, etc.
I hope this helps you differentiate between a BPO and a call center.
VISHAL