Call center is a work place where people either take call 
or make call. taking call is called inbound call center 
where making call is called outbound call center. Tele-
marketing is a type of outbound where people make call to 
sell something.

BPO is a much bigger conception than a call center. BPO 
stands for Business process outsourcing. It is related to 
not only call center. Company can outsource any department 
to an organization. Though generally companies outsource 
their customer support or collection department. Now a days 
companies are out sourcing their varification department 
too. They outsource marketing research, advanced training 
of employees and even Accounting!!! So BPO has much bigger 
aspect than a call center.