Hey Pam,
A call center is an off-shore/on-shore telemarketing office for call support (telemarketing, help desk, front desk, etc.). Since most of us on this thread are from Pakistan, so I will take that up as an example.
If DELL USA would like to run a telemarketing campaign on their laptops but they cannot afford $4000 salary per guy, they would simply outsource the campaign to an off-shore company in Pakistan and request them to carry out the campaign on their behalf. Each individual doing those calls here would cost them